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ILLINOIS  BIOLOGICAL 
MONOGRAPHS 


Vol.1  April,  1915  No.  4 


Editorial  Committee 


Stephen  Alfred  Forbes  William  Trilease 

Henry  Baldwin  Ward 


Published  under  the 

Auspices  of  the  Graduate  School  by 

the  University  of  Illinois 


Copyright,  1915 
By  the  University  of  Illinois 


SOME    NORTH     AMERICAN 
LARVAL  TREMATODES 


WITH  EIGHT  PLATES 


BY 


WILLIAM  WALTER  CORT 


Contributions   from  the 

Zoological   Laboratory   of   the   I'niversity   of   Illinois   under   the   direction    of 

Henry   B.   Ward,    No.   44 


THESIS 

Submitted  in  Partial  Fulfilment  of  the  Requirements 

for  the  Degree  of  Doctor  of  Philosophy 

in  Zoology  in  the  Graduate  School 

of  the  University  of  Illinois 

1914 


TABLE  OF  CONTENTS 

.        .  PAGE 

Introduction 

General  Discussion  7 

Forms  Previously  Described  from  North  America 8 

Methods   of    Study 9 

Description   of   Species 
Monostome  Cercariae 

Cercaria  urbancnsis  Cort   1914 11 

Knoystment  of  Cercaria   urbancnsis 12 

Redia  of  Cercaria  urbancnsis 13 

Mature  Cercaria  urbancnsis  14 

Aniphistome  Cercariae  17 

Cercaria  inhabilis  Cort   1914 18 

Redia  of  Cercaria  inhabilis  21 

Cercaria  diastropha  Cort   1914 22 

Redia  of  Cercaria  diastropha 23 

Critique  of  Cary's  Work  on  Diplodiscus  temporatus 24 

Cercaria  Caryi  Cort  1914. 30 

Distome   Cercariae    31 

Gymnocephalus    Cercariae    31 

Megalurous  Cercariae 

Cercaria  megalura  Cort  1914 31 

Redia  of  Cercaria  megalura  32 

Echinostome   Cercariae   36 

Cercaria  trivohis  Cort  1914 37 

Redia  of  Cercaria  trivolvis  39 

Cercaria  rubra  Cort  1914 40 

Appendix  to  Echinostome  Cercariae 

Cercaria  rcflcxae  Cort  1914 41 

Redia  of  Cercaria  rcflcxae  43 

Microcercous  Cercariae 

Cercaria  trigonura  Cort  1914 44 

Redia  of  Cercaria  trigonura  46 

Furcocercous  Cercariae 

Cercaria  douthitti  Cort  1914 49 

Xiphidiocercariae 52 

Polyadenous  Cercariae 

Cercaria    isocotylea   Cort    1914 53 

Cercaria  polyadcna  Cort   1914 55 

Cercariae  Ornatae  57 

Cercaria    hemilophura   Cort   1914 58 

Microcotylous  Cercariae  59 

Cercaria  leptocantha  Cort  1914 60 

Cercaria  brevicaeca  Cort  1914 61 


The  Classification  of  the  Cercariae 

General  Discussion 62 

Luhe's  Classification  of  the  Cercariae  64 

Lebour's   Classification  of  the  Cercariae 66 

Bibliography    _ 68 

Explanation  of  Plates 71 


453]  LARVAL   TREMATODES—CORT 


INTRODUCTION 

Practically  nothing  is  known  of  the  life-histories  of  the  trematodes 
of  North  America.  Even  in  Europe  where  many  new  adults  are  being 
described  each  year  only  a  few  developmental  cycles  are  completely 
known.    One  reason  for  this  is  to  be  found  in  the  difficulties  involved. 

Two  methods  of  attacking  trematode  life-history  problems  have  been 
employed.  One  is  to  attempt  to  prove  specific  identity  between  cercariae 
and  adults  by  structural  comparisons,  and  the  other  is  to  attempt  to 
find  the  relationship  experimentally.  Positive  results  from  the  first 
method  have  been  few.  The  structure  of  certain  types  of  cercariae  gives 
no  suggestion  of  the  family  to  which  the  adult  belongs,  and  in  the  more 
differentiated  forms  generic  identification  is  very  rarely  possible.  Even 
when  the  comparison  is  narrowed  to  very  similar  cercariae  and  adults 
from  limited  localities  it  is  safest  to  consider  the  results  as  merely  sug- 
gestive. Many  errors  have  crept  into  the  literature  from  too  much  reli- 
ance on  this  method.  In  some  cases  the  mere  suggestion  of  probable 
identity  by  one  author  has  been  taken  by  another  as  if  it  were  an  estab- 
lished fact.  It  is  only  when  the  adult  has  been  experimentally  raised 
from  the  cercariae  or  the  larvae  from  the  eggs  of  the  adult  that  the 
establishment  of  specific  identity  can  be  made  sure.  The  experimental 
method  of  study  also  has  its  dangers.  The  factors  involved  are  so  com- 
plex and  so  little  understood  that  only  the  most  carefully  controlled 
experiments  can  be  considered  as  conclusive.  In  some  very  recent  work 
as  well  as  in  older  papers  larvae  and  adults,  shown  later  to  be  entirely 
unrelated,  have  been  joined  experimentally. 

In  connection  with  efforts  to  solve  developmental  problems  it  is 
often  argued  that  given  the  structure  of  a  cercaria,  it  is  possible  to 
draw  conclusions  as  to  the  environment  where  it  encysts,  the  life  it 
leads,  etc.  Further,  that  if  the  course  of  development  of  one  member 
of  a  group  is  known,  it  can  be  concluded  that  the  others  follow 
the  same  lines.  Dollfus  (1914)  finds  evidence  against  both  generaliza- 
tions. He  states  that  after  a  comparative  examination  of  cercariae  and 
the  environment  in  which  they  live,  he  can  assert  that  cercariae  very 
similar  in  structure  dwell  in  different  hosts  and  have  very  different  kinds 
of  development,  and  that  cercariae  very  different  morphologically  live 


8  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [454 

in  identical  environments  and  have  similar  courses  of  development.  Cer- 
tainly in  the  study  of  the  development  of  digenetic  trematodes  it  will 
be  necessary  to  increase  very  greatly  the  number  of  particular  instances 
known  before  the  induction  of  general  principles  can  safely  go  very 
far. 

In  Europe,  where  more  is  known  of  trematode  development  than 
elsewhere,  the  foundations  of  this  study  were  laid  in  the  middle  of  the 
last  century  by  the  work  of  such  men  as  Leuckart,  Wagener,  Pagen- 
stecker,  La  Valette  St.  George,  de  Filippi,  and  Moulinie.  Altho  they 
made  but  little  progress  in  the  actual  working  out  of  developmental 
cycles,  their  descriptions  of  large  numbers  of  cercariae  from  molluscs 
have  formed  the  basis  for  later  work.  In  England  Nicoll  and  Marie 
Lebour  have  very  recently  made  some  headway  in  the  study  of  larval 
trematodes  altho  as  yet  little  experimental  work  has  been  done.  In  North 
America  only  a  beginning  has  been  made  in  the  study  of  adult  trema- 
todes and  as  yet  there  are  only  a  very  few  scattered  observations  of  larval 
stages.  The  present  work  was  undertaken  by  the  writer  at  the  sug- 
gestion of  Professor  Henry  B.  Ward  as  an  attempt  to  open  up  this  almost 
untouched  field  in  North  America. 

Most  of  the  descriptions  of  larval  trematodes  from  North  American 
molluscs  are  very  inadequate  and  in  many  cases  it  is  impossible  to  tell 
to  which  general  group  of  cercariae  the  forms  belong.  In  only  a  very 
few  instances  are  either  drawings  or  measurements  given.  The  follow- 
ing list  is  an  attempt  to  bring  together  all  references  to  date  on  larval 
trematodes  from  North  American  molluscs. 

1.  Cercaria  hyalocauda  Haldeman  (date?)  also  reported  by  Evarts 
(1880).    Host  Physa  heterostropha  Say.    Locality  (?). 

2.  Cercaria  bUineata  Haldeman  (1840).  Host  Limnea  catascopium, 
Camden,  Delaware. 

3.  Cercariaeum  vagens  (Leidy)    (1847:  220-221) 
Syn.  Distoma  helicis  (Leidy  1847:220-221) 

"         pericardium  Creplin  (1849) 
vagens  Leidy  (1850:  304-310) 
Cercariaeum  helicis  alternatae  Diesing  (1855:398) 
398) 
vagens  Diesing  (1858:42) 
Host  Helix  alternata  and  Helix  albolabris,  Philadelphia,  Pa. 

4.  Cercaria  agilis  Leidy  (1858:  110).  Found  free  in  the  Delaware 
Tiver. 

5.  Monostoma  (Olenocercaria)  lucania  Leidy  (1877:200-201) 
Host  Planorbis  parvus.    Philadelphia,  Pa. 


455]  LARVAL   TREMATODES—CORT  9 

6.  Distoma  (Gymnoccphala)  ascoidca  Leidy  (1877:201).  Host, 
Planorbis  parvus,  Philadelphia,  Pa. 

7.  Distoma  ccntrappendiculatum  Leidy  (1890:416). 
Syn.,  Distoma  appendiculatum  Leidy  (1877:202). 
Host,  Helix  arbora,  Philadelphia,  Pa. 

8.  Distama  cornifrons  Leidy  (1878:382-383).  Host,  Donax  fosser, 
Cape  May,  New  Jersey. 

9.  Distoma  lasium  Leidy  (1890).  Host,  Ilyanassa  obsoleta,  Beach 
Haven,  New  Jersey. 

10.  Ccrcaria  platyura  Leidy  (1890:415-416).  Free  in  a  pool  at 
Fort  Bridger,  Wyoming. 

11.  Cercaria  of  Diplodiscus  temporatus  Stafford,  Cary  (1909). 
Host,  Goniobasis  virginica,  Princeton,  New  Jersey. 

The  present  paper  adds  fourteen  new  species  of  cercariae  from  North 
American  fresh-water  snails.  A  preliminary  report,  taking  up  briefly 
the  structure  and  activity  of  these  cercariae  has  already  been  published 
(Cort,  1914). 


METHODS  OF  STUDY 

To  obtain  this  material  examinations  were  made  of  large  numbers 
of  snails  from  various  localities.  I  want  to  express  my  thanks  to  Dr. 
Ruth  Marshall,  Dr.  M.  F.  Guyer,  Dr.  G.  R.  La  Rue,  Dr.  B.  M.  Allen, 
Dr.  F.  W.  Carpenter,  Dr.  H.  S.  Pratt,  Mr.  A.  F.  Coutant,  Mr.  Hermann 
Douthitt,  Dr.  C.  S.  Mead,  Dr.  A.  Richards,  Dr.  C.  C.  Nutting,  and  Dr. 
J.  E.  Ackert  for  their  kindness  in  collecting  and  shipping  me  living 
snails.  Without  their  aid  it  would  have  been  impossible  to  have  ob- 
tained the  material  for  this  study. 

To  Professor  Henry  B.  Ward,  under  whose  direction  this  work  has 
been  carried  on,  I  wish  to  express  my  appreciation  for  his  interest  and 
helpful  suggestions. 

The  following  method  was  employed  in  the  examination  of  snails 
for  larval  trematodes.  The  shell  was  cut  or  crushed  so  as  to  remove 
the  body,  if  possible,  unbroken.  An  examination  was  then  made  with 
the  low  power  of  the  microscope.  If  larval  trematodes  were  present, 
some  of  them  were  almost  invariably  loosened  from  the  infected  part 
and  scattered  around  the  snail  in  the  water.  The  digestive  gland  was 
the  organ  most  usually  infected,  its  color  being  often  changed  by  the 
pigmentation  of  the  sporocysts  or  rediae.  When  infection  was  found 
a  part  of  the  diseased  tissue  was  preserved  whole  for  sectioning,  an- 


10  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [456 

other  portion  was  teased  apart  to  free  the  larvae  for  preserving  for 
toto  mounts,  and  the  remainder  was  used  for  the  study  of  the  living 
animals.  Much  of  the  anatomy  of  the  cercariae  could  be  made  out 
from  living  specimens.  In  fact  this  proved  to  be  an  extremely  impor- 
tant part  of  the  study,  since  some  points,  for  example  the  smaller 
branches  of  the  excretory  system  and  the  movement,  could  only  be 
observed  in  this  manner. 

For  preservation  of  material  a  number  of  fixatives  were  used.    In 
fixing  the  freed  cercariae  for  toto  mounts  the  best  results  were  obtained 
by  the  use  of  hot  solutions  of  Bouin's  picro-aceto-formol  or  corrosive- 
v  acetic.    For  sectioning  the  infected  organs  were  fixed  in  toto,  since  as 

suggested  by  Cary  (1909:597)  this  is  easier  and  gives  better  results 
than  attempting  to  section  freed  individuals.  A  corrosive-acetic  solu- 
tion was  ordinarily  used  for  fixing  this  material.  Rediae  and  cercariae 
for  toto  mounts  were  stained  in  Mayer's  haemalum,  Delafield's  haema- 
toxylin,  Conklin's  picro-haematoxylin,  and  Mayer's  paracarmine.  The 
specimens  were  as  a  rule  considerably  overstained  in  dilute  solutions 
of  the  stain  and  differentiated  in  HC12  under  the  microscope.  In 
mounting  it  was  found  convenient  to  place  large  numbers  of  the  larvae 
on  one  slide.  The  infected  organs  to  be  sectioned  were  usually  stained 
^  in  bulk  in  Ehrlich's  acid  haematoxylin,  cut  into  sections  5  to  7  micra 
in  thickness,  and  differentiated  on  the  slide,  part  of  them  being  coun- 
terstained  in  eosin. 

On  account  of  their  great  mobility,  small  size,  and  remarkable 
power  of  changing  their  shapes,  some  cercariae  are  very  difficult  objects 
to  study.  No  very  accurate  measurements  can  be  made  of  living  speci- 
mens, and  in  preserved  material  they  are  often  contracted  and  dis- 
torted. With  the  living  cercariae  an  attempt  was  made  to  get  the 
range  of  variability,  and  in  preserved  material,  whenever  possible,  the 
average  measurements  of  a  number  of  well  extended  specimens  were 
taken.  The  measurements  of  preserved  material  are  less  than  those 
from  living  specimens  of  the  same  kind,  even  the  suckers  shrinking 
perceptibly  after  preservation.  For  these  reasons  comparisons  of  the 
cercariae  based  on  size  and  shape,  as  Liihe  (1909:173)  has  suggested, 
are  not  always  by  themselves  very  reliable  criteria  for  specific  deter- 
mination. 


457]  LARVAL   TREMATODES—CORT  11 


MONOSTOME  CERCARIAE 


About  five  per  cent.,  of  the  largest  specimens  of  Physa  gyrina 
from  a  drainage  ditch  north  of  Urbana,  Illinois,  examined  in  December, 
1913,  were  infected  with  the  rediae  and  cercariae  of  a  monostome.  I 
propose  to  name  this  species  Cercaria  urbanensis.  The  infection  was  in 
the  liver  of  the  snail  and  there  were  present  both  rediae  in  different 
stages  of  development  and  free  cercariae.  No  sporocysts  were  found 
and  none  of  the  rediae  contained  rediae.  It  is  interesting  to  note  that 
in  the  descriptions  of  monostome  larvae  no  mention  is  made  of  sporo- 
cysts, and  only  one  observation  of  rediae  developing  from  rediae. 
Braun  (1892:805-806)  notes  that  in  certain  of  the  monostomes  rediae 
are  already  developing  in  the  free  swimming  miracidia,  which  he  con- 
siders to  already  represent  sporocysts.  Looss  (1896:197)  states  that 
in  material  of  Cercaria  imbricata  Looss  collected  from  Bythinia  ten- 
taculata  near  Leipzig,  rediae  were  present  in  which  rediae  were  devel- 
oping. From  this  he  concludes  that  the  life-history  of  this  species  is 
accomplished  in  the  same  manner  as  that  of  the  amphistomes,  which 
have  several  generations  of  rediae. 

When  freed  from  the  liver  of  the  snail  the  redia  had  considerable 
power  of  extension  and  contraction.  The  immature  ones  especially 
stretched  out  the  anterior  end  and  reached  in  all  directions.  No  loco- 
motor appendages  were  present  either  in  young  or  mature  redia  and 
no  locomotion  was  noted.  In  fact  the  redia  of  Cercaria  imbricata  Looss 
is  the  only  monostome  redia  that  is  reported  with  locomotor  append- 
ages. According  to  Looss  (1896:196)  the  appendages  in  this  species 
were  well  marked  in  the  young  redia  but  more  or  less  effaced  with 
advancing  age.  In  some  of  the  largest  rediae  of  Cercaria  urbanensis 
very  peculiar  annular  constrictions  were  noted  in  different  parts  of  the 
body,  which  divided  it  into  two  or  three  separate  regions  connected  by 
very  narrow  passages  (Fig.  12).  These  constrictions  seem  to  be  due 
to  temporary  unequal  contraction  states  in  different  regions  of  the 
circular  body  muscles.  This  condition  for  the  redia  of  Cercaria  urban- 
ensis was  observed  only  in  preserved  material.  Leidy  (1877:200; 
1904:143-144)  notes  such  constrictions  in  the  living  redia  of  a  mono- 
stome cercaria  which  he  calls  Monostoma  lucanica. 


12  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [458 

The  methods  of  locomotion  of  Cercaria  urbanensis,  either  when 
swimming  in  open  water  or  when  upon  a  substratum,  are  very  striking. 
The  body  when  swimming  was  contracted  almost  into  a  round  ball  and 
the  tail,  which  was  curled  ventrally  so  that  it  passed  across  the  ventral 
surface  of  the  body,  lashed  backward  and  forward  with  great  rapidity. 
This  method  of  locomotion  was  very  effective  and  the  cercariae  could 
be  seen  even  with  the  naked  eye  in  rapid  movement  thru  the  water. 
In  spite  of  the  lack  of  a  ventral  sucker  the  cercaria  was  able  to  move 
well  on  a  surface  by  utilizing  two  projections  which  form  the  posterior 
lateral  angles  of  the  body.  In  the  process  of  locomotion  the  cercaria 
took  hold  with  its  oral  sucker  and  the  body  contracted  until  it  was 
practically  round.  Then  the  sucker  let  go  its  hold  and  the  body 
stretched  out,  at  the  same  time  extending  the  posterior  projections 
until  they  became  little  points  digging  into  the  substratum.  Again  the 
oral  sucker  took  hold  and  again  the  body  contracted.  By  a  continued 
repetition  of  these  movements  this  cercaria  progressed  at  about  the  same 
rate  of  speed  as  those  having  two  suckers.  After  each  contraction  the 
posterior  projections  held  the  amount  gained.  During  the  contraction 
and  at  the  beginning  of  the  extension  of  the  body  the  tail  kept  lashing 
very  rapidly,  but  during  the  rest  of  the  movement  it  was  held  still  and 
somewhat  contracted. 

Some  of  the  freed  cercariae  after  moving  around  for  a  while 
settled  down  and  formed  cysts.  A  cercaria  which  was  moving  along 
on  a  surface  extended  and  contracted  its  body  more  and  more  slowly, 
until  while  retaining  a  hold  with  the  oral  sucker  the  body 
became  almost  round.  While  in  this  position  the  cystogenous  material 
was  extruded,  and  the .  appearance  was  soon  given  of  a  round  cyst 
with  a  tail  attached.  Soon  the  worm  inside  freed  itself  from  its  con- 
nection with  the  tail  and  squirmed  around  in  the  cyst.  Finally  the  tail 
wriggled  loose  from  the  cyst  and  continued  swimming  around  for  con- 
siderable time,  resembling  in  form  and  movement  a  free  living  nema- 
tode. The  cysts  formed  in  a  watch  glass  were  flattened  on  the  lower 
surface  and  had  much  the  shape  of  a  chocolate  drop.  The  process  of 
encystment  is  illustrated  by  figures  1,  2,  3,  and  4.  Encystment  in  the 
open  has  been  noted  for  Cercaria  ephemera  Nitzch.  The  encystment 
of  this  species,  which  was  first  reported  by  Nitzch  (1807),  was  the  first 
described  for  a  cercaria.  La  Valette  St.  George  (1855:33-34)  described 
and  figured  the  process  for  the  same  species,  and  in  one  of  his  figures 
shows  the  tail  attached  to  the  cyst.  Von  Linstow  (1896:377)  for  Cer- 
caria monistomi  von  Linstow  describes  the  encystment  in  the  same  host 
as  the  larval  generation. 


459]  LARVAL   TREMATODES—CORT  13 

The  rediae  of  Cercaria  itrbancnsis  (Figs.  10,  11,  and  14)  were  in 
various  stages  of  development,  varying  in  length  from  0.52  mm.  to 
1.08  mm.  and  in  width  from  0.12  mm.  to  0.22  mm.  In  shape  they 
are  elongate  sac-like  forms  smallest  at  the  anterior  and  widest  a  short 
distance  in  front  of  the  posterior  extremity.  The  mouth  is  at  the  an- 
terior tip  and  the  pharynx  varies  with  the  size  of  the  animal.  In  the 
smallest  redia  studied  which  had  a  length  of  0.52  mm.  the  pharynx 
was  0.038  mm.  in  length  by  0.048  mm.  in  width  while  in  one  1.08  mm.  in 
length  it  was  0.07  mm.  by  0.08  mm.  The  passage  thru  the  pharynx 
immediately  opens  into  the  intestine  which  is  at  first  narrow  but  soon 
widens  greatly.  In  all  stages  of  development  the  intestine  is  propor- 
tionally very  large,  having  a  diameter  of  from  one-third  to  two-thirds 
the  width  of  the  body  and  reaching  to  within  0.08  mm.  to  0.16  mm. 
of  the  posterior  end.  It  is  rather  clear  and  transparent  and  contains 
only  a  small  amount  of  food  material  floating  in  a  considerable  quantity 
of  fluid.  The  outer  cuticula  of  the  redia  is  very  thin  and  can  hardly 
be  distinguished  as  a  separate  layer.  Inside  of  this  is  a  very  strong 
layer  of  circular  muscles  which  encircle  the  body  as  separate  strands, 
each  about  0.0018  mm.  in  width  and  about  the  same  distance  apart. 
The  strands  of  the  longitudinal  layer  are  very  thin  and  only  visible 
under  favorable  conditions.  Inside  of  the  muscle  layers  are  several 
rows  of  parenchymatous  cells  with  rather  large  nuclei.  These  cells  do 
not  form  a  circumscribed  lining  of  the  body  cavity  but  are  irregular 
and  in  young  rediae  where  the  lining  is  several  cells  thick,  strands 
extend  from  them  thruout  the  body  cavity  forming  an  irregular  net- 
work (Fig.  13).  Into  this  loosely  filled  space  between  the  body  wall 
and  the  intestine  the  cercariae  push.  In  the  oldest  rediae  the  wall 
has  been  reduced  to  one  layer  and  the  movements  of  the  cercariae  have 
broken  down  the  parenchymatous  strands  and  converted  the  region 
between  the  wall  and  the  intestine  into  a  well  defined  cavity,  which 
is  more  or  less  completely  filled  with  developing  cercariae.  In  the 
posterior  end  of  the  redia  is  the  germ  gland,  in  front  of  this  are  germ 
balls  and  further  forward  in  the  older  rediae  are  differentiated  cercariae. 
In  the  youngest  redia  (Fig.  10)  studied  the  developing  embryos  were 
all  back  of  the  middle  of  the  body  and  the  furtherest  developed  was  a 
mass  of  embyronic  cells  about  0.09  mm.  in  length  and  0.06  in  width 
and  having  no  tail.  In  none  of  the  rediae  was  the  body  cavity  crowded 
with  cercariae,  there  being  but  two  or  three  well  developed  forms,  and 
in  one  mature  specimen  there  were  no  differentiated  cercariae  present 
leaving  the  body  cavity  empty  for  about  two-thirds  of  its  length.  These 
conditions  and  the  fact  that  the  oldest  cercariae  in  the  rediae  are  not 
fully  mature  and  that  matured  and  almost  matured  cercariae  are  found 


14  ILLIXOIS  BIOLOGICAL  MONOGRAPHS  [460 

free  in  the  livers  of  the  infected  snails,  show  that  the  cercariae  as 
they  develop  are  continually  making  their  way  out  of  the  rediae  to 
finish  their  growth  in  the  liver  of  their  host.  The  most  highly  differ- 
entiated cercaria  found  in  the  redia  (Fig.  11,  14,  cr)  was  0.3.  mm.  in 
length  and  0.1  mm.  in  width  and  had  a  tail  0.22  mm.  in  length.  The 
two  lateral  eye-spots  were  developed  and  the  pigment  was  present 
to  a  considerable  extent  around  them,  but  was  not  further  scattered 
or  concentrated  to  form  the  so-called  intermediate  eye-spot.  Outlines 
of  the  longitudinal  vessels  of  the  excretory  system  and  the  anlage  of 
the  reproductive  system  could  be  clearly  distinguished.  The  oral  sucker 
was  well  defined  but  none  of  the  rest  of  the  digestive  system  could 
be  distinguished. 

The  mature  Cercaria  urbanensis  (Fig.  5)  varies  greatly  in  shape 
being  at  greatest  contraction  nearly  round,  about  0.27  mm.  in  length 
and  0.20  mm.  in  width  and  when  extended  0.54  mm.  long  and  0.11  mm. 
wide.  When  not  in  motion  the  tail  is  contracted  being  about  0.2  mm. 
long  and  0.05  mm.  wide  at  its  base.  At  times  of  greatest  movement 
the  tail  becomes  attenuated  to  about  one-half  its  usual  diameter  and 
often  reaches  a  length  of  1.2  mm.  It  is  weakly  attached  to  the  dorsal 
mid-line  of  the  posterior  end  of  the  body  and  tapers  to  a  sharp  point. 
There  is  an  inverse  ratio  between  the  contraction  of  the  body  and  the 
tail  for  when  the  body  is  most  contracted  the  tail  is  most  extended  and 
vice  versa. 

In  view  of  the  great  power  of  movement  of  Cercaria  urbanensis 
the  histological  structure  of  its  tail  is  of  considerable  interest.  Just 
inside  of  the  thin  cuticula  is  a  layer  of  circular  muscles  arranged  as 
strands  separated  by  about  twice  their  length  from  each  other.  Inside 
of  this  is  a  very  strong  layer  of  longitudinal  muscle  fibers  each  0.0026 
mm.  in  diameter.  Next  comes  a  single  layer  of  parenchymatous  cells 
somewhat  irregularly  elongated  with  nuclei  0.005  mm.  in  diameter.  Ex- 
tending the  length  of  the  tail  and  forming  a  core  are  two  rows  of  long 
cells  which  are  close  together  and  have  their  long  axes  parallel  with  the 
length  of  the  tail.  These  cells  vary  in  size  having  a  length  from  0.028 
mm.  to  0.035  mm.  In  a  cross  section  of  a  tail  0.075  mm.  wide  one  of 
these  cells  measured  0.026  mm.  in  thickness  and  0.014  mm.  in  width.  They 
are  full  of  heavily  staining  granules  and  their  nuclei  are  0.007  mm. 
in  diameter.  There  is  nothing  suggestive  of  a  possible  function  for 
these  cells.  Figure  9,  a  cross  section  thru  a  tail,  shows  the  structures 
described  above. 

At  the  posterior  lateral  angles  of  the  body  are  projections  which 
may  be  extended  to  aid  the  animal  in  locomotion.  The  tips  of  these 
projections  are  made  firm  by  structures  which  appear  to  be  infoldings 


461]  LARVAL   TREMATODES—CORT  15 

of  the  outer  cuticula.  One  of  the  structures  (Fig.  8)  is  ovoid  with 
two  cuticular  walls,  having  a  loosely  fitted  space  between  them,  and 
a  very  narrow  central  cavity  communicating  with  the  exterior  by  a 
small  pore  at  the  tip  of  the  projection.  They  apparently  have  no  suck- 
ing action,  since  no  muscles  are  present  and  the  central  cavity  con- 
tracts when  the  projection  is  extended.  They  evidently  have  a  function 
in  locomotion  analogous  to  setae.  Similar  locomotor  projections  have 
been  described  for  Cercaria  ephemera  Nitzch  and  Cercaria  imbricata 
Looss,  and  for  Leidy's  (1877:200)  (1904:143-144)  Monostoma  lucania 
from  North  America  they  are  merely  mentioned  as  conical  projections. 
Ssinitzin  (1905,  Plate  4,  figs.  75  and  76)  figures  these  structures 
in  Cercaria  ephemera  Nitzch  as  clearly  circumscribed  projections  with 
a  considerable  cavity  lined  with  spines.  Figures  6a  and  b  are  copies 
of  his  figures.  Certainly  the  structures  figured  by  Ssinitzin  differ  con- 
siderably from  those  of  Cercaria  urbanensis.  Marie  Lebour  (1907:442) 
in  the  monostome  cercaria  from  Paludestrina  stagnalis  which  she  con- 
siders to  be  Cercaria  ephemera  describes  the  posterior  locomotor  pro- 
jections as  sucker  like  structures,  which  are  circular  in  outline  and 
divided  in  two  by  a  bar.  Her  figure  shows  them  as  structures  quite 
comparable  to  those  of  Cercaria  urbanensis  but  very  different  from* 
Ssinitzin 's  figures.  She  assigns  no  especial  reason  for  considering  them 
to  be  sucking  structures.  The  posterior  locomotor  projections  as  described 
by  Looss  (1896:194)  for  Cercaria  imbricata  offer  still  greater  differences. 
The  cavity  is  comparatively  large  with  but  one  cuticular  wall  and 
divided  at  its  center  into  two  parts  by  a  projection.  Figure  7  is  a 
copy  of  Looss',  Plate  14,  Figure  151. 

Mature  specimens  of  Cercaria  urbanensis  are  heavily  pigmented 
especially  at  the  anterior  end,  and  have  two  lateral  pigmented  eyes 
with  lenses  and  a  central  anterior  spot  which  is  formed  by  a  condensa- 
tion of  pigment.  The  eyes  are  situated  dorsad  at  each  side  of  the  large 
esophageal  commissure  and  above  the  obtuse  angles  formed  by  the 
large  nerves  which  pass  forward  and  backward.  Each  true  eye  is 
formed  by  a  mass  of  pigment  in  the  form  of  a  cup  the  bottom  of  which 
is  thicker  than  the  sides.  A  lens  fits  into  the  opening  of  the  cup,  leav- 
ing a  space  between  its  lower  surface  and  the  bottom  of  the  cup.  An 
eye  spot  has  a  diameter  0.017  mm.  and  the  depth  of  the  pigment  cup 
is  0.024  mm.  At  the  anterior  end  surrounding  each  eye  are  scattered 
pigment  granules  extending  in  all  directions  and  about  as  clearly  defined 
ventrally  as  dorsally.  Anteriorly  a  condensation  forms  the  so-called 
anterior  eye  spot,  and  scattered  granules  reach  well  beyond  the  limit 
of  the  oral  sucker.     Posteriorly  the   pigment  granules   become   more 


16  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [462 

scattered  and  continue  as  two  longitudinal  lines  to  the  posterior 
end  of  the  body  on  each  side  of  the  excretory  bladder  (Fig.  5, 
p.  1).  These  lines  of  pigment  extend  thru  the  whole  dorsal  ventral 
thickness  of  the  body  and  are  as  apparent  on  one  side  as  on  the  other. 
Along  each  edge  and  appearing  ventrally  are  two  other  irregular  lines 
of  pigment  which  do  not  extend  as  far  back.  Little  pigment  flecks 
are  scattered  from  these  lines  out  thru  the  body.  In  the  young  cercaria 
the  pigment  develops  first  around  the  eyespots  in  dense  masses  and 
spreads  gradually  with  growth  both  forward  and  backward  along  the 
lines  mentioned.  It  is  only  in  the  oldest  cercaria  that  it  is  spread  suffi- 
ciently to  form  the  anterior  spot  and  the  lines  extending  to  the  posterior 
end. 

The  digestive  system  (Fig.  5)  of  Cercaria  urbanensis  is  like  that 
described  for  other  monostome  cercariae.  The  mouth  at  the  anterior 
tip  is  sub-ventral  and  the  oral  cavity  is  surrounded  by  a  relatively  small 
oral  sucker,  averaging  0.043  mm.  in  length  and  0.049  mm.  in  width. 
The  esophagus  is  very  narrow  and  0.05  mm.  to  0.06  mm.  long  depending 
on  the  contraction.  The  cecal  bifurcations  are  close  together  at  their 
beginnings  but  soon  spread  further  apart.  They  are  always  within  and 
slightly  dorsad  of  the  large  longitudinal  excretory  vessels  and  extend 
almost  to  the  end  of  the  body.  The  ceca  are  not  yet  functional,  being 
composed  of  a  solid  mass  of  cells. 

The  excretory  system  (Fig.  5,  ex)  is  typical  of  the  group.  At  the 
posterior  end  is  the  excretory  bladder  opening  just  below  the  base 
of  the  tail.  From  this  extends  forward  two  large  vessels  which  unite 
in  the  midline  just  back  of  the  oral  sucker.  Thruout  their  whole  course 
they  are  filled  with  small  round  concretions  which  disappear  in  the 
process  of  preservation.  It  is  very  difficult  to  be  positive  of  the  rela- 
tions of  the  excretory  system  of  the  tail  but  as  nearly  as  they  can  be 
made  out  they  are  as  follows:  No  openings  could  be  found  in  the 
tail  and  a  single  vessel  passed  forward  from  near  the  tip  becoming 
larger  nearer  the  body  and  opening  at  the  excretory  pore. 

Almost  the  whole  body  of  Cercaria  urbanensis  is  filled  when  mature 
with  large  unicellular  cystogenous  glands  containing  small  granules. 
Only  the  very  anterior  tip,  the  posterior  locomotor  projections  and  the 
tail  are  free  from  them. 

Only  a  few  monostome  cercariae  have  been  recognized.  All  of  these 
except  Cercaria  lophocera  Filippi  (1857:5)  correspond  very  closely  in 
structure  to  Cercaria  urbanensis.  Cercaria  imbricata  Looss  is  distin- 
guished by  the  fact  that  the  rediae  have  smaller  intestines  and  lateral 
appendages  and  by  the  structure  of  the  posterior  locomotor  projections 
of  the  cercaria   (Looss  1896:192-197).     Several    different    forms    have 


463]  LARVAL   TREMATODES—CORT  17 

probably  been  described  as  Cercaria  ephemera  as  it  seems  improbable 
that  Ssinitzin  and  Lebour  have  described  the  same  form.  The  only 
difference  that  can  be  definitely  determined  by  comparing  the  descrip- 
tions of  Cercaria  ephemera  with  Cercaria  urbane  mis  is  in  the  structure 
of  the  posterior  locomotor  appendages  of  the  cercaria.  In  von  Linstow  ?s 
(1896:376-377)  description  of  Cercaria  monostomi  both  rediae  and  cer- 
cariae  are  larger  than  in  any  of  the  other  species,  the  arrangement 
of  pigment  is  different  from  that  in  Cercaria  urbane  mis,  and  no  poste- 
rior locomotor  appendages  are  described.  These  may  have  been  over- 
looked as  they  are  very  small  and  not  easily  seen  unless  the  animal  is 
studied  alive.  Cercaria  lophocera  described  by  Filippi  (1857:5)  from 
Italy  is  entirely  different  from  all  other  monostomes  known. 

Two  descriptions  of  monostome  cercariae  have  been  made  from 
North  America.  Leidy  (1877:200-201)  describes  as  M onostoma  lucanica, 
a  form  from  Planorbis  parvus.  Unfortunately  not  enough  detail  is  given 
to  make  the  comparison  possible.  The  other  form  is  described  by  Halde- 
man  as  Cercaria  hyalocauda.  It  has  been  impossible  to  find  Haldeman's 
original  description.  Evarts  (1880)  describes  this  species  from  Physa- 
heterostropha.  Altho  so  little  detail  is  given  that  a  minute  comparison 
cannot  be  made  between  this  species  and  Cercaria  urbanensis,  certain 
points  can  be  definitely  made  out.  Cercaria  hyalocauda  is  about  half 
again  as  large  as  Cercaria  urbanensis,  and  its  cyst  is  much  larger  than 
that  of  the  latter  species,  being  0.32  mm.  to  0.20  mm.  It  seems  evi- 
dent that  the  two  forms  are  not  identical. 

Altho  monostome  cercariae  have  been  known  since  1817,  the  life- 
history  of  no  one  of  them  has  been  proven  experimentally.  Looss  (1896  :- 
192-193)  argues  from  distribution  and  structural  correspondence  that 
Cercaria  imbricata  from  Egypt  is  the  larval  form  of  Notocotyle  triseri- 
ale  from  the  duck.  Liihe  (1909:178)  suggests  that  Cercaria  ephemera 
is  the  larval  form  of  either  Notocotyle  triserialc  or  Catatropis  verrucosa 
(Frol.),  but  cannot  belong  to  Typhlocoelum  flavum  (Mehl.)  on  account 
of  differences  in  the  digestive  systems  of  the  two  forms.  So  little  is 
known  of  the  monostomes  of  the  United  States  that  it  is  useless  to  gen- 
eralize in  regard  to  the  life-history  of  Cercaria  urbanensis. 

AMPHISTOME     CERCARIAE 

Amphistome  cercariae  of  two  species  were  collected  from  speci- 
mens of  Planorbis  trivolvis  from  three  localities.  Two  snails  out  of 
eighteen  from  Lawrence,  Kansas  had  the  livers  infected  with  rediae 
and  very  large  pigmented  cercariae.  Out  of  large  numbers  of  Planorbis 
trivolvis  examined  from  around  Urbana,  Illinois,  one  from  a  small  pond 


18  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [464 

was  infected  with  this  same  form.  The  second  of  these  species,  a 
smaller  unpigmented  cercaria,  was  found  in  one  of  twenty  specimens 
of  Planorbis  trivolvis  from  a  small  pond  in  the  suburbs  of  Chicago. 
In  all  the  infected  snails  mature  and  immature  cercariae  were  found 
free  in  the  liver,  the  mature  forms  being  nearest  the  periphery,  and 
the  active  rediae  contained  no  fully  developed  cercariae.  There  were 
no  sporocysts  present  and  no  rediae  in  which  rediae  were  develop- 
ing, and  in  none  of  the  infected  snails  were  rediae  or  cercariae  numerous. 
Since  the  large  pigmented  species  is  very  unwieldy  in  movement,  I 
propose  to  name  it  Cercaria  inhabilis,  and  the  smaller  species  on  account 
of  the  way  in  which  it  changes  its  body  shape  will  be  given  the  name, 
Cercaria  diastropha. 

Cercaria  inhabilis  swam  sluggishly  in  open  water.  It  contracted 
its  body  and  lashed  its  tail  backward  and  forward,  moving  in  an 
unwieldy,  irregular  fashion.  In  fact  the  body  was  too  large  in  propor- 
tion to  the  size  of  the  tail  for  rapid  locomotion.  On  a  substratum 
the  cercaria  extended  and  contracted  the  body  but  was  unable  to  move 
by  the  aid  of  its  suckers. 

When  in  a  state  of  average  contraction,  about  that  of  figure  16, 
the  body  of  Cercaria  inhabilis  is  pear  shaped,  tapering  in  the  anterior 
half,  and  wider  but  of  uniform  diameter  posteriorly.  It  is  the  largest 
of  the  cercariae  studied,  having  an  average  length  in  mounted  speci- 
mens of  0.8  mm.  and  a  width  of  0.4  mm.  The  thickness  is  a  little 
greater  than  half  the  width.  The  oral  sucker  is  elongate,  0.16  mm.  in 
length  and  0.12  mm.  in  width  with  the  retrodorsal  pharyngeal  pockets 
which  are  characteristic  of  some  amphistomes.  The  acetabulum  is  very 
large  averaging  0.23  mm.  in  diameter;  it  is  at  the  posterior  end  of 
the  body  and  is  turned  ventrad. 

Two  large  eye-spots  are  present  just  back  of  the  pharynx.  They 
are  located  from  one-fourth  to  one-third  of  the  distance  from  the  anterior 
to  the  posterior  end,  and  in  a  specimen  0.27  mm.  wide  at  this  region, 
they  were  0.065  mm.  from  the  outer  margins  and  0.13  mm.  apart.  These 
eyes  are  composed  of  the  lens  and  the  cone  of  pigment  like  those 
already  described  for  the  monostome,  Cercaria  urbanensis.  Figure  17, 
a  section  thru  the  eyes,  shows  them  in  their  relation  to  the  nervous 
system  and  other  adjacent  structures. 

In  the  development  of  Cercaria  inhabilis  the  pigment  starts  in  the 
eyes  and  is  deposited  first  in  a  peculiar  way  over  most  of  the  anterior 
half  of  the  body.  In  the  youngest  cercariae  found  outside  of  the 
rediae  very  little  pigment  is  seen  and  that  found  near  the  eyes 
(Fig.  18,  p.).     In  forms  a  little  older  (Fig.  19,  p.),  the  pigment  has 


465]  LARVAL   TREMATODES—CORT  19 

begun  to  spread  out  a  little  forward  and  to  the  sides,  but  for  the  most 
part  backward  along  two  irregular  lines.  At  first  all  the  pigment 
strands  were  connected  in  an  irregular  way,  but  soon  at  the  ends  of 
the  lines  little  flecks  were  scattered  out.  Backward  along  the  lines 
the  pigment  becomes  thicker  (Fig.  20,  p)  and  collects  at  points  of  union 
in  the  network  masses.  At  an  older  stage,  (Fig.  21,  p)  the  pigment 
strands  from  the  two  lateral  lines  become  connected  and  form  an  irreg- 
ular network  about  the  middle  of  the  oral  sucker,  laterad  as  far  as  the 
sides,  and  backward  to  about  the  middle  of  the  body.  As  the  cercariae 
become  older  the  masses  and  lines  of  pigment  break  up  and  are  more 
scattered.  In  the  mature  specimen  (Fig.  16)  the  pigmentation  shows 
as  scattered  brownish  flecks  extending  thru  the  whole  thickness  of  the 
body  (Fig.  17,  p)  and  reaching  from  the  oral  sucker  back  to  the  middle. 

The  cystogenous  glands  in  Cercaria  inhabilis  are  as  thickly  devel- 
oped dorsally  as  ventrally,  and  extend  from  the  oral  sucker  to  the 
acetabulum.  Viewed  from  the  surface  they  appear  as  small  rounded 
bodies,  0.012  to  0.016  mm.  in  diameter,  filled  with  rod-shaped  cysto- 
genous granules.  Figure  17  shows  them  to  be  elongate,  unicellular,  club- 
shaped  glands  with  small  nuclei. 

The  tail  of  Cercaria  inhabilis  varies  from  one-third  to  greater  than 
the  body  length.  It  is  attached  to  the  tip  of  the  body  above  the  acetab- 
ulum and  is  easly  lost  in  free  swimming  animals.  The  cuticula  of  the 
tail  is  very  thin  and  no  trace  could  be  found  of  circular  muscles.  The 
longitudinal  muscle  layer,  however,  is  conspicuous  and  is  formed  of  a 
series  of  strands  each  0.0035  mm.  in  thickness,  which  extend  from  the 
base  to  the  tip.  Inside  of  the  muscles  is  a  layer  of  irregular  paren- 
chymatous cells,  with  nuclei  0.005  to  0.007  mm.  in  diameter,  and  irreg- 
ular indistinct  cell  boundaries.  The  space  between  this  layer  and  the 
excretory  tubule,  which  courses  down  the  center  of  the  tail,  is  filled 
with  large  cells  with  faintly  granular  cytoplasm,  and  large  nuclei,  0,008 
to  0.009  mm.  in  diameter.  These  cells  are  similar  to  those  forming  the 
core  of  the  tail  of  Cercaria  urbanensis,  but  inclose  no  such  darkly  stain- 
ing granules.  Several  of  these  cells  may  occur  in  one  cross  section  since 
they  do  not  seem  to  be  arranged  in  regular  rows.  The  central  excretory 
tubule  of  the  tail  has  a  considerable  diameter,  and  contains  in  its  walls 
very  large  scattered  nuclei,  0.01  to  0.012  mm.  in  diameter.  A  cross 
section  of  the  tail  (Fig.  22)  shows  these  structures. 

A  comparison  of  the  nucleoli  of  the  different  nuclei  of  the  tail  of 
Cercaria  inhabilis  is  interesting.  Of  the  three  kinds  of  nuclei  present 
viz.  1.  nuclei  of  the  excretory  tubule,  2.  nuclei  of  the  large  central 
cells,  3.  parenchymatous  nuclei,  the  first  two  have  large  very  clearly 
defined  nucleoli  and  little  if  any  chromatin  scattered  outside  of  it.    Of 


20  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [466 

these  two  the  second  is  the  larger.  In  the  parenchymatous  nuclei  the 
nucleoli  are  quite  small,  not  definite  in  outline,  and  much  chromatin 
is  scattered  thru  the  nucleus. 

The  excretory  vessel  in  the  tail  of  Cercaria  inhabilis  is  a  large 
single  tube  running  down  almost  to  the  tip,  where  it  divides  to  open 
to  each  side  (Fig.  16).  This  vessel  becomes  narrow  as  it  passes  from 
the  tail  to  the  body  but  immediately  widens  into  the  excretory  bladder, 
which  is  a  triangular  vesicle  dorsad  and  anteriad  to  the  acetabulum. 
No  vessels  or  flame  cells  were  made  out  in  the  acetabulum.  Into 
each  corner  of  the  bladder  open  vessels  which  were  traced  only  as  far 
as  the  eye-spots.  In  these  vessels  were  present  the  round  highly  refrac- 
tive concretions,  already  noted  in  the  excretory  system  of  Cercaria 
urbanensis  and  by  Looss  in  the  amphistome  cercariae  described  by 
him  (Looss,  1896:181). 

The  mouth  of  Cercaria  inhabilis  is  at  the  anterior  tip  of  the  oral 
sucker  which  juts  out  slightly  from  the  body.  The  anterior  margin 
of  the  mouth  cavity  is  smooth  or  only  slightly  roughened.  There  are 
no  papillae  present  around  the  margin  of  the  oral  cavity  such  as  Looss 
(1896:179)  describes  for  the  amphistome  cercaria  from  Egypt,  which 
he  considers,  to  be  the  larva  of  Gastrodiscus  egyptiacus.  The  mouth 
opens  into  an  oral  cavity,  from  which  a  narrow  passage  runs  directly 
backword;  this  changes  into  the  esophagus  at  the  posterior  limit  of 
the  oral  sucker.  Into  each  side  of  the  oral  cavity  open  the  blind  tubes 
of  the  retrodorsal  pharyngeal  pockets  (Fig.  16,  pp.).  The  oral  cavity 
has  a  length  of  0.059  mm.  and  the  lateral  blind  tubes  are  0.086  mm. 
in  length.  As  it  passes  out  of  the  oral  sucker  the  digestive  tube  becomes 
the  thin,  straight-walled  estfphagus,  0.021  mm.  in  caliber,  which  has  a 
length  of  0.1  to  0.15  mm.  depending  on  the  state  of  contraction  of 
the  cercaria.  Just  before  the  esophagus  bifurcates  it  is  reinforced  by 
a  mass  of  circular  muscle  fibers  of  a  sphincter,  making  a  characteristic 
structure  which  superficially  resmbles  a  pharynx.  This  pseudo-pharynx 
is  about  as  long  as  wide,  being  0.038  mm.  in  diameter,  and  has  a  wall 
0.009  mm.  thick,  composed  of  from  12  to  14  separate  layers  of  mus- 
cles. Longitudinal  muscles  were  not  made  out  either  in  this  region  or 
in  the  esophagus.  The  inner  lining  of  the  esophagus  is  non-cellular 
and  tiny  projections  extend  out  into  its  lumen  (Fig.  17,  es).  After 
bifurcation  the  intestinal  ceca  run  laterally  for  a  short  distance  and 
then  turn  posteriad  to  reach  within  0.05  mm.  to  0.06  mm.  of  the  anterior 
margin  of  the  acetabulum.  The  ceca  are  fairly  wide,  0.028  mm.  to 
0.036  mm.,  and  the  inner  walls  contain  flattened  nuclei  which  jut  out 
a  little  into  their  lumina.  Figure  16  illustrates  the  relations  of  the 
digestive  system. 


467]  LARVAL   TREMATODES—CORT  21 

In  Ccrcaria  inhabilis  the  anlage  of  the  reproductive  organs  begins 
to  take  definite  shape.  It  is  composed  of  dense  masses  of  small  heavily 
staining  nuclei.  In  the  largest  cercariae  four  areas  connected  by  lines 
are  marked  out.  At  the  middle  line  of  the  body  just  back  of  the  bifurca- 
tion of  the  intestinal  ceca,  is  an  elongate  mass  which  reaches  up  very 
close  to  the  ventral  surface.  This  is  probably  the  anlage  of  the  ends 
of  the  ducts  of  the  reproductive  system,  leading  up  to  the  genital  pore. 
Slightly  back  of  this  and  close  together  are  two  masses,  the  primordia 
of  the  testes,  and  further  back  is  the  largest  densest  mass  which  repre- 
sents the  ovary  and  its  surrounding  structures  (Fig.  16,  ra). 

Along  with  the  cercariae  in  the  livers  of  the  infected  snails  were 
numbers  of  active  radiae  all  in  about  the  same  stage  of  development 
(Fig.  15).  "When  they  were  freed  from  the  snail  they  were  very  mobile 
extending  and  contracting  and  making  some  progress  even  on  the  smooth 
surface  of  the  watch  glass.  There  were  two  pairs  of  locomotor  append- 
ages and  the  posterior  extremity  was  attenuated  and  pointed.  The 
anterior  pair  of  locomotor  appendages  was  just  back  of  the  posterior 
limit  of  the  intestine  and  at  normal  extension  the  second  pair  was 
about  the  same  distance  back.  The  tail  region  was  shorter  and  more 
slender.  These  proportions  varied  greatly  with  the  contraction  of  the 
animal.  In  alcoholic  material  the  locomotor  appendages  of  the  redia 
are  often  obliterated  by  the  contraction  of  the  muscles. 

One  of  the  largest  of  these  rediae  (Fig.  15)  measures  1.36  mm. 
in  length  and  0.31  mm.  in  width,  and  the  posterior  limit  of  the  volumin- 
ous intestine  is  0.56  mm.  from  the  anterior  tip.  The  pharynx  is  very 
small  in  proportion  to  the  size  of  the  body,  measuring  0.086  mm.  in 
length  and  0.065  mm.  in  width.  The  intestine  contains  dark  brown 
materal  evidently  from  the  liver  of  the  host.  Bunched  around  the 
anterior  tip  of  the  redia  just  back  of  the  oral  sucker  are  elongate  uni- 
cellular glands  of  the  type  often  found  in  rediae,  which  send  forward 
ducts  to  open  near  the  tip. 

The  cuticula  and  muscle  layers  of  the  rediae  of  Cercaria  inhabilis 
are  quite  thin.  The  circular  muscles  are  the  strongest  but  do  not  show 
externally.  Inside  of  the  muscles  except  in  the  region  of  the  anterior 
part  of  the  intestine  and  the  germ  gland,  the  wall  is  made  up  of  a  layer 
of  cells  which  is  thin  in  the  older  forms.  In  none  of  the  rediae  are 
embryos  much  differentiated,  the  largest  showing  mere  stumps  of  tails, 
the  beginning  of  suckers  and  traces  of  eye-spots.  Since  immature  cer- 
cariae are  found  outside  of  the  redia  it  is  evident  that  they  make 
their  way  out  at  a  very  early  stage,  and  complete  their  development 
free  in  the  snail's  liver. 


22  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [468 

Since  Cercaria  diastropha  resembles  Cercaria  inhabilis  closely  the 
description  will  be  limited  to  pointing  out  the  differences.  As  in  Cer- 
caria inhabilis  different  stages  of  development  of  the  cercaria  are  found 
free  in  the  snail.  The  mature  cercariae  extended  and  contracted  their 
bodies  very  actively  but  none  were  noted  swimming  freely.  This  may 
have  been  due  to  the  condition  of  the  material  studied.  There  was  no 
cheek  on  the  one  examination. 

Cercaria  diastropha  (Fig.  23)  is  cylindrical  in  cross  section,  pointed 
anteriorly,  and  when  not  contracted  or  flattened  the  region  in  front 
of  the  acetabulum  is  but  little  wider  than  that  sucker.  In  living  speci- 
mens the  body  varied  from  0.27  to  0.54  mm.  in  length  according  to  the 
state  of  contraction,  and  the  width  changed  from  0.20  to  0.08  mm.  The 
tail  is  always  shorter  than  the  body,  being  0.22  mm.  to  0.38  mm.  in 
length  and  with  an  average  width  near  its  base  of  0.054  mm.,  The  tail 
is  attached  dorsad  to  the  acetabulum,  which  is  terminal  and  forms  a 
flattened  base  for  the  conical  body  (Fig.  24).  The  oral  sucker  is  elong- 
ate and  in  a  specimen  of  average  contraction  has  a  length  as  great  or 
greater  than  the  acetabulum.  In  an  animal  about  the  state  of  con- 
traction of  Figure  23  the  oral  sucker  had  a  length  of  0.11  mm.  and  a 
width  of  0.065  mm.  and  the  acetabulum  had  a  diameter  of  0.105  mm. 

The  eye-spots  are  like  those  already  described  but  are  larger  in 
proportion  to  the  size  of  the  body  than  in  Cercaria  inhabilis.  Except 
for  a  very  limited  area  around  the  eyes  Cercaria  diastropa  is  entirely 
unpigmented. 

The  body  is  filled  with  cystogenous  glands  from  the  oral  sucker  to 
the  acetabulum. 

On  account  of  the  freedom  from  pigmentation  it  was  possible  to 
work  out  the  excretory  system  further  in  Cercaria  diastropha  than  in 
Cercaria  inhabilis.  Figure  23,  ex,  shows  the  relations  of  this  system. 
The  tail  vessel  and  the  bladder  are  alike  in  both  forms.  The  much 
convoluted  crura  of  Cercaria  diastropha  which  are  large  and  contain 
scattered  concretions,  extend  as  unbranched  vessels  up  to  the  region 
of  the  eyes.  There  they  receive  small  branches  from  all  parts  of  the 
body.  It  was  possible  to  trace  the  largest  of  these  branches  altho  on 
account  of  the  cystogenous  glands,  the  flame  cells  and  the  smallest 
ducts  were  not  found.  Into  the  tips  of  the  crura  open  on  each  side 
two  vessels,  one  from  the  side  of  the  oral  sucker  and  one  from  the 
posterior  end.  The  posterior  branch  is  soon  divided  into  an  outer  and 
an  inner  vessel,  which  subdivide  to  reach  all  parts  of  the  posterior  body 
region. 


469]  LARVAL   TREMATODES—CORT  23 

The  digestive  system  in  Cercaria  diastropha  is  similar  to  that  of 
Cercaria  inhabilis,  but  the  oral  sucker  and  its  pouches  -are  larger  in 
proportion  to  the  size  of  the  acetabulum,  and  the  intestinal  ceca  reach 
nearer  the  posterior  end  of  the  body. 

In  Cercaria  diastropha  the  anlage  of  the  reproductive  organs  is 
further  developed  than  in  Cercaria  inhabilis.  It  is  differentiated  into 
four  clearly  separated  areas,  which  bear  the  same  general  relation  to 
the  adult  organs  as  in  the  other  species.  They  are  not  however  con- 
nected with  strands  of  nuclei  and  are  not  in  exactly  the  same  relative 
position  as  in  Cercaria  inhabilis. 

The  above  descriptions  show  that  Cercaria  inhabilis  and  Cercaria 
diastropha  differ  considerably  in  the  size  and  shape  of  the  body,  the  ratio 
in  size  of  the  suckers  and  in  the  position  of  the  acetabulum,  the  amount 
of  pigmentation,  and  in  the  anlage  of  the  reproductive  organs. 

Rediae  of  Cercaria  diastropha  (Fig.  25)  were  in  different  stages 
of  development.  No  rediae  were  found  in  which  other  rediae  were 
developing  and  in  none  were  the  cercariae  further  dveloped  than  in  the 
rediae  of  Cercaria  inhabilis.  The  rediae  were  very  mobile,  having  remark- 
able power  of  changing  their  shape,  and  with  the  aid  of  the  posterior 
locomotor  appendages  could  move  fairly  well.  In  the  younger  rediae 
the  body  would  sometimes  be  extended  to  five  or  six  times  the  length 
when  contracted.  This  mobility  is  correlated  with  the  extereme  devel- 
opment of  the  circular  muscles,  which  show  clearly  as  annular  bands 
(Fig.  25,  ab).  In  the  young  living  redia  part  of  the  excretory  system 
could  be  made  out.  One  longitudinal  trunk  from  the  anterior  extremity 
was  traced  until  it  met  two  trunks  from  the  posterior  end. 

In  the  youngest  redia  studied  there  were  practically  no  germ 
balls  in  the  body  cavity ;  the  length  of  a  toto  mount  slightly  contracted 
was  0.45  mm.  and  the  greatest  width  0.13  mm.  The  intestine  extended 
to  a  point  0.22  mm.  from  the  anterior  end  and  was  relatively  voluminous. 
The  oral  sucker  had  a  length  of  0.43  mm.  and  a  width  of  0.032  mm. 
One  of  the  largest  specimens  measured  0.78  mm.  in  length  and  0.16 
mm.  in  width.  The  intestine  reached  one-third  the  body  length.  The 
oral  sucker  was  about  the  same  size  as  in  the  younger  specimens,  the 
length  being  0.044  mm.  and  the  width  0.038  mm.  From  the  above 
description  it  is  evident  that  the  redia  of  Cercaria  diastropha  is  very 
much  like  that  of  Cercaria  inhabilis.  The  greatest  difference  is  in  a 
greater  mobility  correlated  with  a  greater  development  of  the  circular 
muscles  in  the  former. 

Altho  a  large  number  of  adult  trematodes  belonging  to  the  family 
Paramphistomidae  has  been  described,  I  have  found  in  the  literature 


24  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [470 

mention  of  only  three  cercariae  belonging  to  this  group.  The  cercaria 
of  Diplodiscus  subclavatus  has  been  known  for  a  long  time,  having  been 
first  described  by  de  Filippi  and  best  described  by  Looss  (1892 :162-166). 
Of  the  two  other  forms  one  was  first  described  by  Sonsino  (1892)  as 
Cercaria  pigmentata,  and  later  shown  experimentally  by  Looss  ( 1896  :- 
185-191)  to  be  the  larval  form  of  Amphistomum  conicum  (Paramphis- 
tomum  cervi).  The  other  was  also  described  by  Looss  (1896:177-185) 
as  the  larva  of  Gastrodiscus  aegyptiacus.  This  conclusion,  however,  only 
rests  on  the  structural  comparison  of  the  cercaria  and  the  adult. 

The  five  amphistome  cercariae  now  known  belong  to  two  different 
sub-families  of  the  Paramphistomidae.  The  cercaria  of  Paraphistomum 
cervi  differs  from  the  others  in  lacking  the  pockets  of  the  oral  sucker,  and 
in  having  a  connection  between  the  longitudinal  crura  of  the  excretory 
system.  It  belongs  to  the  sub-family  Paramphistominae.  The  other  four 
of  these  cercariae  are  much  alike  and  belong  to  the  sub-family  Diplodis- 
cinae.  They  all  have  the  retrodorsal  pockets  of  the  oral  sucker,  and 
the  muscular  enlargement  of  the  esophagus  at  the  bifurcation  of  the 
intestinal  ceca.  Cercaria  diastropha  differs  considerably  from  the  other 
three  in  its  small  size,  lack  of  pigmentation  and  in  the  proportionally 
large  size  of  the  oral  sucker.  The  cercaria  of  Diplodiscus  subclavatus, 
that  of  Gastrodiscus  aegyptiacus  and  Cercaria  inhabilis  are  very  similar. 
The  first  of  these  differs  from  the  other  two  in  the  large  size  of  the 
pharynx  of  its  redia  and  in  the  small  size  of  the  intestine.  Cercaria 
inhabilis  is  larger  than  Looss'  cercaria  of  Gastrodiscus  aegyptiacus.  My 
measurements  show  that  the  oral  sucker  of  this  species  is  twice  as 
large  as  his,  and  the  acetabulum  is  very  much  larger.  There  is  noth- 
ing in  Cercaria  inhabilis  to  correspond  to  the  papillae  found  by  Looss 
around  the  mouth  of  his  form,  and  the  intestine  of  the  redia  is  much 
larger  in  his  species. 

The  only  adult  trematode  which  I  have  found  in  the  literature 
from  the  United  States  which  resembles  these  cercariae  in  structure  is 
Diplodiscus  temporatus  Stafford.  This  form  has  the  retrodorsal  pharyn- 
geal pockets  found  in  the  cercariae  and  also  the  muscular  thickening 
of  the  esophagus.  The  arrangement  of  the  reproductive  organs  is  such 
that  those  of  the  adult  might  be  derived  from  the  anlage  of  either  of 
the  above  cercariae.  As  far  as  conclusions  from  comparative  structure 
are  concerned  either  Cercaria  inhabilis  or  Cercaria  diastropha  might 
be  the  larvae  of  Diplodiscus  temporatus.  Infection  experiments  alone 
can  clear  up  this  point.  It  is  evident,  that  Cary  (1909)  is  mistaken 
in  the  larval  form  which  he  assigns  to  Diplodiscus  temporatus. 

Cary  (1909)  described  as  belonging  to  the  life-history  of  Diplodis- 
cus temporatus  Stafford  sporocysts  and  rediae,  both  containing  cercariae 


471]  LARVAL   TREMATODES—CORT  25 

from  Goniobasis  virginica  obtained  near  Princeton,  New  Jersey.  In  1911 
Cary  sent  me  some  of  the  material  which  he  had  used  in  the  prepara- 
tion of  this  paper,  including  specimens  of  Diplodiscus  temporatus  from 
his  experimental  tadpoles.  A  study  of  this  material  and  a  careful  anal- 
ysis of  Cary's  account  has  convinced  me  that  he  has  described  in  this 
paper,  two  different  species  of  larval  trematodes  neither  of  which 
belong  to  Diplodiscus  temporatus. 

The  snails  of  the  species  Goniobasis  virginica  which  he  collected 
from  the  Delaware  and  Raritan  canal  near  Princeton  in  the  fall  of  1908, 
contained  rediae  in  which  cercariae  were  developing,  but  those  collected 
from  the  same  locality  in  the  spring  of  the  following  year  and  those 
from  the  Delaware  river  near  Trenton,  contained  sporocysts  in  which 
cercariae  were  developing.  Cary  assigns  both  these  stages  unhesitat- 
ingly to  the  same  species,  for  no  other  reason  so  far  as  can  be  judged 
than  that  they  were  collected  from  the  same  species  of  snail  from  the 
same  general  locality.  That  in  the  same  species  of  trematode,  cercariae 
should  be  found  developing  from  both  sporocysts  and  rediae  is  without 
parallel.  Further  in  his  own  descriptions  Cary  shows  that  he  is  deal- 
ing with  two  separate  types  of  larvae.  In  connection  with  his  account 
of  the  development  of  the  cercariae  in  the  sporocysts,  (p.  643),  he 
writes  of  the  cercaria. 

"In  the  dorsal  part  of  the  sucker  (oral  sucker)  there  is  developed 
the  dart  (Stachel).  This  lies  in  a  thin  structureless  sheath  between 
the  muscle  cells.  It  is  shaped  like  a  short  arrow  with  a  comparatively 
broad  head." 

Neither  in  the  description  nor  in  the  figure  (Cary,  1909,  PL  30, 
Fig.  6)  of  the  cercaria  which  develops  from  a  redia  is  a  dart  shown. 
Further  a  comparison  of  the  cercariae  developing  from  rediae  with  the 
others  developing  from  sporocysts  from  the  material  which  Cary  sent 
me,  shows  that  they  are  entirely  different  in  practically  every  character. 
Figures  26  and  27  are  drawings  made  to  scale  of  these  two  types  of 
cercariae.  According  to  Liihe's  (1909)  classification  of  the  cercariae 
these  forms  would  fall  into  two  entirely  unrelated  groups.  The  smaller 
one  with  the  boring  spine  which  develops  in  the  sporocysts  (Fig.  26) 
very  evidently  belongs  in  the  Xiphidiocercariae  (Liihe,  1909:189-200) 
while  the  larger  form  agrees  with  the  characteristics  of  the  Gymno- 
cephalous  cercariae  (Liihe,  1909:182-186).  That  two  such  diverse  cer- 
cariae should  develop  into  the  same  adult  is  utterly  impossible.  Since 
in  his  infection  experiments  Cary  uses  only  the  cercariae  which  develops 
from  rediae,  he  certainly  can  have  no  evidence  that  the  cercariae 
which  develops  from  sporocysts  and  are  entirely  different  from  the 
first  type  have  any  connection  whatever  with  Diplodiscus  temporatus. 


26  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [472 

Therefore  Cary's  whole  discussion  in  the  embrological  part  of  the  paper 
(pp.  617-647)  which  is  based  on  the  study  of  the  sporocysts  and  the 
cercariae  developing  in  them  cannot  without  further  evidence  be  given 
a  place  in  the  life-history  of  Diplodiscus  temporatus.  Since  it  proves 
the  thesis  that  Cary  sets  out  to  make  that  the  embryo  in  the  sporocysts 
develops  from  parthenogentic  eggs,  this  account  is  a  very  important 
contribution  to  trematode  embrology. 

Another  point  in  Cary's  paper  which  is  very  striking  is  the 
great  difference  between  the  cercaria  which  he  describes  as  belonging 
to  Diplodiscus  temporatus  and  other  amphistome  cercariae,  especially 
that  of  Diplodiscus  suoclavatus,  of  which  Looss  has  worked  out  the 
life-history  (Looss,  1892).  A  comparison  of  these  two  forms  shows 
striking  differences  in  the  shape  and  size  of  the  body,  in  the  size  and 
position  of  the  acetabulum,  in  the  excretory  system  especially  that  of 
the  tail,  and  in  their  activities  and  encystment.  (Compare  Cary,  1909, 
PL  30,  Fig.  6  and  Looss,  1892,  PL  20,  Fig.  20).  That  cercariae  belong- 
ing to  the  same  genus  should  be  so  different  in  structure  is  contrary 
to  all  the  accepted  views  of  trematode  development.  The  few  observa- 
tions found  in  the  literature  on  the  life-histories  of  closely  related 
species  of  trematodes  show  a  closer  structural  correspondence  betweeen 
the  cercariae  than  the  adults. 

A  study  of  Cary's  material  shows  that  he  is  in  error  in  the  descrip- 
tion and  drawing  of  the  largest  of  the  two  cercariae  in  certain  funda- 
mental points.  In  his  drawing  (plate  30,  Fig.  6)  and  description  the 
digestive  system  of  the  cercaria  corresponds  to  that  of  the  adult  Diplo- 
discus temporatus,  in  having  pharyngeal  pouches  and  in  the  muscular 
enlargement  of  the  esophagus  at  the  point  of  division  into  the  intestinal 
ceca.  In  his  material  the  cercaria  has  no  pharyngeal  pouches  and  the 
clearly  circumscribed  pharynx  which  is  followed  by  an  enlarged  portion 
of  the  esophagus  is  entirely  different  from  his  description  and  drawing 
for  this  form.  Compare  figure  27  and  Plate  30,  Figure  6  of  Cary's 
paper.  In  fact,  the  digestive  system  of  his  so-called  cercaria  of  Diplo- 
discus temporatus  does  not  in  reality  correspond  to  that  of  the  adult 
as  his  drawing  and  description  suggest.  Cary  is  also  in  error  in  his 
description  of  the  tail  of  this  cercaria,  since  the  material  which  he 
sent  me  shows  that  it  is  much  longer  than  he  figures  it  and  has  a 
truncated  end  (Fig.  27).  Neither  does  the  reproductive  anlage  of  this 
cercaria  agree  with  his  description  and  drawing  (p.  606,  PL  30,  Fig.  6), 
for  instead  of  three  definitely  circumscribed  areas  two  in  front  of  the 
acetabulum  and  one  behind,  it  really  consists  of  a  small  mass  just  in 
front  of  the  excretory  vesicle  connected  with  a  mass  in  front  of  the 
acetabulum  by  a  line  of  nuclei. 


473]  LARVAL   TREMATODES—CORT  27 

When  the  adult  Diplodiscus  temporatus  is  compared  with  the  larger 
of  the  cercariae  which  Cary  assigns  to  it  certain  differences  of  structure 
are  noted  so  fundamental  that  it  seems  impossible  that  the  two  forms 
can  be  the  same  species.  Compare  figures  27  and  28.  The  adult  is  a 
typical  amphistome  with  the  conical  body  terminating  in  a  very  large 
acetabulum,  while  the  cercaria  is  widest  toward  the  anterior  end,  flat- 
tened and  its  acetabulum  which  is  only  a  little  larger  than  the  oral 
sucker  is  just  back  of  the  center  of  the  body.  Certainly  very  remark- 
able changes  in  shape  and  position  of  the  organs  would  be  necessary 
before  the  cercaria  which  developed  in  rediae  in  Goniobasis  virginica, 
which  Cary  described,  could  metamorphose  into  an  adult  Diplodiscus 
temporatus.  Cary  makes  no  attempt  to  bridge  this  gap  altho  it  would 
seem  from  his  infection  experiments  that  intermediate  stages  should  have 
been  obtained.  It  is  especially  hard  to  believe  that  the  ventral  sucker 
could  have  migrated  from  the  middle  of  the  body  to  the  posterior 
end  and  have  become  so  much  larger  in  proportion  to  the  oral  sucker, 
and  that  the  digestive  system  could  have  changed  so  fundamentaly 
as  would  have  been  necessary.  In  those  species  of  trematodes  in  which 
the  development  is  known  the  digestive  and  excretory  systems  are  very 
much  alike  in  the  cercariae  and  in  the  adults.  The  principal  changes 
come  in  the  development  of  the  reproductive  organs  and  the  correspond- 
ing enlargement  of  body  regions,  usually  the  post-acetabular. 

The  infection  experiments  that  Cary  conducted  to  prove  the  con- 
nection between  this  cercaria  and  Diplodiscus  temporatus  seem  con- 
vincing until  they  are  carefully  analyzed.  To  carry  conviction  they 
should  have  been  better  controlled,  described  in  more  detail  and  the 
stages  of  development  worked  out.  That  I  may  do  Cary  no  injustice 
I  will  quote  in  full  his  account  of  the  experiments  that  he  used  to 
prove  the  connection  between  the  cercaria  and  the  adult  (Cary,  1909: 
612-1613). 

"On  Oct.  30  a  number  of  tadpoles  of  Rana  catcsbiana  were  secured 
and  several  put  in  each  jar  containing  infected  snails.  The  tadpoles 
came  from  a  pond  in  the  grounds  of  the  Biological  Hall  of  the  Univer- 
sity of  Pennsylvania,  where  Goniobasis  is  not  found,  so  it  seemed  improb- 
able that  they  would  be  infected  with  the  parasite  that  was  found  about 
Princeton.  As  an  added  precaution  all  of  the  tissues  of  three  of  the 
tadpoles  were  carefully  examined  without  finding  parasites  of  any  kind. 
When  the  tadpoles  had  been  for  a  week  in  the  jar  containing  the 
encysted  cercariae,  a  dead  individual  was  found.  This  one  was  exam- 
ined for  the  presence  of  parasites  with  the  following  results:  Nine 
worms,  which  from  the  condition  of  the  sexual  organs  could  be  recog- 
nized as  young,  were  found  in  the  intestine  of  this  tadpole.    The  other 


28  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [474 

organs  of  the  body  were  entirely  free  from  parasites.  The  worms  in 
the  intestine  were  about  2.5  mm.  in  length  and  1  mm.  in  diameter  at 
the  posterior  end.  All  of  them  were  found  in  the  last  third  of  the 
intestine  of  the  tadpole,  scattered  throut  that  part  of  its  length.  Dur- 
ing the  time  that  they  were  under  examination,  as  was  also  true  in. 
every  other  instance,  the  worms  remained  attached  to  the  intestine  of 
the  host  by  a  large  posterior  sucker.  The  anterior  part  of  the  body 
was  in  almost  constant  motion. 

"Others  of  the  tadpoles  used  in  the  experiment  died  from  time  to 
time  and  were  examined  for  the  presence  of  worms.  The  result  of  the 
examination  was  identical  in  everj^  case.  A  greater  or  less  number  of 
worms,  ranging  from  ten  to  thirty-one,  was  found  in  the  posterior  part 
of  the  intestine.  In  no  case  were  any  of  the  worms,  or  any  other  recog- 
nizable parasites,  found  in  any  of  the  other  organs  of  the  tadpoles. 

1 '  The  intestines  of  two  of  the  tadpoles  at  the  time  of  death  contained 
worms  still  within  the  cyst.  Among  the  others,  individuals  of  different 
ages  could  be  recognized  so  it  was  definitely  established  that  the  tad- 
poles could  serve  as  the  host  for  the  sexually  mature  worms. 

' '  Since  it  seemed  probable  from  the  number  of  worms  found  in  each 
of  the  dead  tadpoles  that  they  had  in  many  instances  been  the  direct 
cause  of  the  death  of  the  host,  some  larger  tadpoles  were  secured  for 
further  experiments.  These  were  put  into  jars  containing  encysted 
cercariae;  but  after  they  had  remained  there  for  a  few  days  they  were 
transferred  to  a  jar  in  which  there  had  been  no  snails.  A  tadpole  from 
this  jar  was  killed  each  week  to  note  the  development  of  the  parasites. 
The  conditions  of  the  environment  proved  unfavorable  for  the  tadpoles 
and  the  last  one  of  them  died  on  Jan.  19,  1909,  after  having  been 
infected  with  Diplodiscus  nine  weeks.  The  worms  which  were  taken 
from  the  intestine  of  this  tadpole  had  fully  developed  sexual  organs, 
but,  so  far  as  could  be  determined  from  the  condition  of  the  jar,  no 
eggs  had  been  laid,  or  at  least  no  embryos  had  been  developed." 

An  analysis  of  these  experiments  shows  a  number  of  weaknesses. 
The  fact  that  the  tadpoles  came  from  a  pond  in  the  grounds  of  the 
Biological  Hall  at  the  University  of  Pennsylvania  offers  no  check  on 
the  results.  The  location  of  the  pond  at  Philadelphia  and  not  at  Prince- 
ton and  the  fact  that  Goniobasis  does  not  occur  there,  can  hardly  be 
called  evidence  that  Diplodiscus  temperatus  is  not  present  in  great  num- 
bers. The  only  check  that  Cary  gives  on  his  experiments  is  the  exam- 
ination for  parasites  of  three  tadpoles  out  of  the  whole  lot.  These  he 
reports  free  from  all  parasitic  infection.  This  does  not  prove  that  the 
others  were  uninfected.  The  three  examined  may  have  been  without 
parasites  while  the  rest  were  infected,  or  it  is  even  possible  that  the 


475]  LARVAL   TREMATODES—CORT  29 

parasites  may  have  been  overlooked.  Any  man  even  if  somewhat  accus- 
tomed to  examinations  for  parasites  is  likely  to  overlook  them,  especially 
if  they  are  small  and  the  infection  light,  until  a  chance  finding  directs 
his  attention  to  a  particular  organ.  Therefore  it  would  seem  that  the 
above  experiments  were  not  sufficiently  controlled  to  prove  that  no  infec- 
tion of  Diplodiscus  temporatus  was  present  in  the  tadpoles  previous  to 
the  experimental  feeding. 

For  infection  he  puts  the  tadpoles  in  the  jars  with  infected  snails 
and  when  a  week  later  one  of  the  tadpoles  proved  to  be  infected  with 
nine  immature  specimens  of  Diplodiscus  temporatus  he  concluded  that 
they  had  developed  from  the  larvae  in  the  snails.  A  comparison  of 
the  structure  of  the  two  forms  shows  how  improbable  this  is.  The 
largest  cercaria  of  this  type  in  the  alcholic  material,  which  Cary  sent 
me,  had  a  body,  0.40  mm.  in  length  and  0.20  mm.  in  width  and  an 
acetabulum  0.065  mm.  in  diameter.  The  only  measurements  that  Cary 
gives  in  his  description,  which  were  evidently  taken  from  a  living  speci- 
men, are  2  mm.  in  length  for  both  body  and  tail  and  0.15  mm.  in  width. 
The  tail  is  usually  at  ieast  as  long  as  the  body,  so  that  would  make 
the  body  of  the  cercaria  not  over  0.1  mm.  in  length.  Cary  gives  no 
measurements  for  the  acetabulum  but  his  drawing  (PI.  30,  Fig.  6)  shows, 
it  less  than  half  the  width  of  the  body. 

Since  the  cercariae  must  have  had  about  the  size,  shape,  and  propor- 
tions given  above  at  the  time  this  tadpole  was  supposed  to  have  eaten 
the  cysts,  their  metamorphosis  surely  must  have  been  extraordinary  to 
have  developed  in  a  week  into  the  immature  specimens  of  Diplodiscus 
temperatus,  which  Cary  found  in  the  tadpoles.  He  describes  these  forms 
as  2.5  mm.  in  length,  1  mm.  in  diameter  at  the  posterior  end,  and  as 
being  attached  by  the  typical  large  posterior  sucker.  The  posteror 
sucker  of  the  young  Diplodiscus  temporatus  (Fig.  28)  has  practically 
the  width  of  the  posterior  end,  which  in  this  case  would  be  almost  1  mm. 
Therefore  if  Cary's  contention  be  correct  his  cercariae  in  one  week 
almost  tripled  their  length,  changed  the  whole  shape  of  the  body,  and 
increased  their  width  five  times.  The  acetabulum  must  in  some  way 
have  jumped  from  the  center  of  the  body  to  the  posterior  extremity 
and  increased  its  diameter  ten  times.  Since  such  a  transformation  is 
impossible  one  is  forced  to  conclude  that  the  tadpoles  used  in  the  experi- 
ments were  already  infected  with  Diplodiscus  temporatus,  and  that 
there  is  no  connection  between  this  species  and  the  cercariae  used  in  the 
experiment.  The  fact  that  according  to  Cary  every  tadpole  examined 
was  infected  with  Diplodiscus  temporatus  cannot  be  taken  as  attesting 
the  success  of  the  experiments  but  merely  the  general  uniformity  of 
the  original  infection.     A  detailed   comparison  of  figures  27  and  28 


30  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [476 

shows  how  fundamental  are  the  differences  between  the  cercariae  used  in 
the  experiments  and  Diplodiscus  temporatus. 

The  second  experiment  is  even  less  convincing  than  the  first  because 
no  check  whatever  is  given  and  the  source  of  the  tadpoles  is  not  indi- 
cated, altho  it  is  perhaps  to  be  taken  for  granted  that  they  are  obtained 
from  the  same  source  as  the  first  batch.  These  tadpoles  were  killed  each 
week  to  note  the  development  of  the  parasites  and  stages  of  Diplodiscus 
temporatus  were  found.  If  the  development  had  followed  the  course 
that  the  writer  maintains  it  would  have  been  possible  with  this  material 
to  find  transitional  stages  in  change  of  shape,  supposed  migration  of  the 
acetabulum,  etc.  This  Cary  has  not  done  and  even  the  possibility  seems 
to  have  escaped  his  notice,  since  it  is  not  mentioned  in  the  paper. 

The  following  points  have  been  proved  in  the  above  discussion. 

1.  That  Cary  described  two  entirely  different  species  of  cercariae 
as  belonging  to  Diplodiscus  temporatus; 

a.  Those  with  stylets,  which  develop  in  sporocysts, 

b.  Larger  forms  without  stylets  which  develop  in  rediae. 

2.  That  since  the  second  type  only  were  used  in  the  infection  ex- 
periments, no  connection  between  the  first  type  and  Diplodiscus  tem- 
poratus can  have  been  shown. 

3.  That  the  infection  experiments  were  not  sufficiently  controlled 
to  be  conclusive. 

4.  That  the  cercariae  used  could  not  have  possibly  developed  into 
Diplodiscus  temporatus,  since  the  two  forms  differ  so  fundamentally  in 
structure. 

Since  the  stylet  form  of  Cary's  two  species,  which  I  have  named 
Cercaria  caryi,  is  very  small,  and  no  living  material  is  available,  it  does 
not  seem  wise  at  the  present  time  to  attempt  a  detailed  description. 
Figure  26  gives  the  most  salient  features.  From  the  presence  of  the 
Stylet,  the  small  number  of  the  stylet  glands,  and  the  small  size  of  the 
acetabulum  it  may  be  placed  with  the  Xiphidiocercariae  in  Liihe's  group 
of  the  Cercariae  microcotylae  (Liihe,  1909:  196-198). 

I  was  fortunate  enough  to  obtain  further  material  of  the  larger 
form,  so  that  a  detailed  description  of  it  is  possible.  On  account  of  the 
great  length  to  which  the  tail  is  sometimes  stretched  I  shall  describe  it 
as  Cercaria  megalura. 


477]  LARVAL   TREMATODES—CORT  31 


DISTOME  CERCARIAE 

The  great  bulk  of  known  cercariae  belong  to  this  division.  In  ray 
material  are  eleven  distome  cercariae  representing  eleven  of  the  principal 
sub-groups. 

GYMNOCEPHALOUS   CERCARIAE 

Since  beyond  the  fact  that  they  develop  in  rediae  the  cercariae 
placed  in  this  subdivision  agree  only  in  the  absence  of  certain  characters, 
it  is  without  doubt  an  unnatural  group.  However  in  the  present  state  of 
our  knowledge  it  is  convenient  to  retain  it.  Of  my  material  only  Cer- 
caria  megalura  belongs  here.  This  species  and  its  allies  differ  so  much 
from  all  the  other  cercariae  of  the  gymnocephalous  group,  that  I  pro- 
pose to  make  them  the  basis  of  a  new  sub-group,  to  which  the  name 
Megalurous  or  heavy-tailed  cercariae  may  be  given. 

MEGALUROUS    CERCARIAE 

From  73  specimens  of  Pleurocera  elevatum  from  the  Sangamon 
river  near  Mahomet,  Illnois,  examined  during  November  and  December, 
1913,  one  was  found  with  the  liver  packed  with  rediae  in  which  devel- 
oped a  very  peculiar  kind  of  cercaria.  Comparison  showed  this  form 
to  be  the  same  as  the  larger  cercaria  which  Cary  assigns  to  Diplodiscus 
temporatus.  Since  Cary's  account  is  not  very  complete,  obscured  by  a 
mistaken  viewpoint,  and  incorrect  in  many  particulars,  a  further  de- 
scription of  this  species  seems  advisable.  Altho  many  of  the  cercariae 
seemed  fully  matured,  none  were  found  free  in  the  organs  of  the  snail 
and  no  rediae  were  found  which  contained  rediae. 

Living  rediae  and  cercariae  of  Cercaria  megalura  moved  actively. 
The  redia  was  very  active  and  the  region  back  of  the  posterior  locomotor 
appendages  on  account  of  its  mobility  and  attenuation  resembled  a  tail. 
The  anterior  portion  of  the  body  also  could  be  extended  and  contracted 
freely  and  with  the  aid  of  the  locomotor  appendages  locomotion  was 
possible.  The  cercaria  was  unable  to  use  its  tail  for  swimming  in  open 
water  but  on  a  substratum  it  moved  fairly  rapidly  with  the  aid  of  the 
suckers.  With  the  acetabulum  attached  the  anterior  end  would  reach 
out  and  the  oral  sucker  take  hold.    The  acetabulum  would  then  loosen 


32  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [478 

its  grip,  arid  the  body  contract  until  the  suckers  were  close  together. 
The  acetabulum  would  again  take  hold  just  back  of  the  oral  sucker, 
which  would  in  its  turn  become  loosened  and  extended.  Locomotion 
consisted  in  a  continued  repetition  of  these  movements.  During  this 
process  the  tail  was  contracted  and  took  no  part  in  the  movement.  At 
certain  times  a  cercaria  became  attached  by  the  posterior  tip  of  the  tail, 
which  is  furnished  with  an  adhesive  organ.  The  animal  then  extended 
to  five  or  six  times  its  usual  length,  and  became  greatly  attenuated. 
While  in  this  position  the  cercaria  moved  continually  with  a  wriggling 
\/  motion.    Looss  (1896:202)  noted  a  similar  activity  in  Cercaria  distoma- 

tosa  Sonsino  from  Cleopatra  bidimoidcs  Bourg.,  from  Cairo,  Egypt,  a 
form  very  much  like  Cercaria  megalura.  This  cercaria  became  attached 
by  the  extremity  of  the  tail  to  the  surface  of  the  water  or  to  some  bodies 
such  as  plants  or  branches  of  trees  very  near  the  surface  of  the  water, 
and  moved  in  a  serpentine  manner  like  a  tubificid  worm.  This  compari- 
son would  apply  equally  well  to  Cercaria  megalura.  What  relation  this 
peculiar  habit  has  to  the  future  development  of  the  cercaria  is  not 
known. 

No  cercariae  were  found  encysting  altho  large  numbers  had  ex- 
truded cystogenous  material  in  the  form  of  a  sort  of  open  tube  around 
the  body  (Fig.  30).  In  fact  this  seemed  to  be  the  normal  procedure 
when  the  animal  came  in  contact  with  fresh  water.  Cary  (1909:609- 
610)  performed  experiments  with  some  of  his  specimens  of  Cercaria 
megalura  which  showed  that  this  extrusion  of  cystogenous  material  was 
due  to  the  change  from  the  conditions  in  the  liver  of  the  snail  to  fresh 
water.  Looss  (1896:201-203)  found  no  free  individuals  of  Cercaria 
distomatosa  in  which  the  cystogenous  material  was  still  in  the  glands, 
and  he  also  mentioned  that  when  the  cercariae  were  taken  up  in  a  pipette 
they  became  encysted  as  quick  as  a  flash.  He  noted  further  that  they 
encysted  also  in  the  open,  loosening  their  hold  and  dropping  to  the 
bottom  after  having  lived  for  a  period  free. 

The  redia  (Figs.  31  and  36)  of  Cercaria  megalura  is  an  elongate  sac 
slightly  tapering  toward  the  anterior  end,  with  the  posterior  locomotor 
appendages  about  six-sevenths  of  the  distance  from  the  anterior  to  the 
posterior  extremity.  It  is  widest  just  in  front  of  the  appendages  and 
tapers  almost  to  a  sharp  point  posteriorly.  The  birth  pore  is  on  the 
dorsal  side  just  back  of  the  pharynx. 

The  mouth  of  the  redia  is  at  the  anterior  tip  and  the  oral  cavity 
opens  into  a  short,  narrow  passage,  which  widens  out  almost  immediately 
into  the  voluminous  digestive  tract.  This  extends  back  of  the  posterior 
locomotor  appendages  nearly  to  the  end  of  the  body.  In  cross  sections 
(Fig.  35)  the  intestine  occupies  from  one-third  to  two-thirds  of  the  body 


479]  LARVAL   TREMATODES—CORT  33 

cavity  depending  on  the  amount  of  food  material  present  and  the 
pressure  from  the  developing  cercariae.  In  the  inner  lining  of  the 
intestine  were  found  flattened,  scattered  nuclei,  but  no  cell  boundaries 
could  be  distinguished.  The  body  cavity  occupies  most  of  the  entire 
region  from  just  back  of  the  oral  sucker  up  to  the  posterior  tip,  but 
does  not  extend  into  the  posterior  locomotor  appendages  and  the  tail-like 
posterior  extremity.  These  regions  are  filled  with  parenchymatous 
tissue  in  which  definite  cell  boundaries  could  be  determined  (Fig.  35). 
The  wall  of  the  body  cavity  of  even  the  youngest  redia  is  very  thin,  and 
in  the  inner  lining  of  flattened  pavement  cells  with  flattened  nuclei,  the 
cell  boundaries  could  be  distinguished  only  with  difficulty.  A  small 
germ  gland  consisting  of  but  few  differentiated  cells  is  present  at  the 
posterior  extremity  of  the  body  cavity. 

All  ages  of  rediae  were  present  in  the  snail  from  those  in  which 
the  oldest  contained  cercaria  was  scarcely  differentiated  at  all,  to  those 
in  the  body  cavities  of  which  there  were  from  four  to  eight  almost  fully 
matured  cercariae.  The  youngest  redia  studied  (Fig.  36)  which  con- 
tained no  cercariae  having  the  cystogenous  glands  at  all  developed,  was 
0.53  mm.  in  length  and  0.12  mm.  in  greatest  width.  The  posterior  loco- 
motor appendages  were  0.097  mm.  from  the  posterior  end,  and  the 
intestine  extended  to  within  0.076  mm.  of  the  posterior  extremity.  The 
pharynx  was  slightly  elongated,  being  0.049  mm.  in  length  and  0.043 
mm.  in  width.  The  body  cavity  contained  a  number  of  developing 
embryos  only  one  of  which  was  far  enough  along  to  be  recognized  as  a 
cercaria.  In  this  embryo  (Fig.  36,  cv)  the  tail  was  a  mere  stub  hardly 
marked  off  from  the  body.  The  length  of  the  body  was  0.33  mm.  and  its 
width  0.054  mm.,  while  the  tail  had  a  length  of  0.038  mm.  and  a  width 
of  0.032  mm.  The  sucker,  digestive  system,  and  reproductive  anlage 
could  be  made  out. 

From  one  of  the  largest  rediae  studied  the  following  measurements 
were  taken  (Fig.  31).  The  length  was  1.16  mm.,  and  the  width  0.19 
mm.  The  posterior  locomotor  appendages  were  0.27  mm.  from  the  pos- 
terior extremity,  and  the  intestine,  which  filled  about  half  the  body 
cavity,  extended  to  within  0.086  mm.  of  the  posterior  tip.  There  were 
four  fully  matured  cercariae,  one  of  which  while  still  in  the  redia  had 
extruded  its  cystogenous  material,  and  a  number  of  developing  embryos 
of  all  ages.  The  pharynx  of  this  redia  was  0.054  mm.  in  length,  and 
0.049  mm.  in  width,  showing  almost  no  development  in  size  from  the 
very  youngest  redia. 

The  length  and  width  of  the  body  of  Cercaria  megalura  (Figs.  29 
and  30)  vary  greatly  with  the  state  of  contraction.  The  tail  varies 
from  one-half  the  length  of  the  body  when  the  animal  is  moving  on  a 


34  ILLIXOIS  BIOLOGICAL  MONOGRAPHS  [480 

substratum  to  ten  times  that  length  when  the  cercaria  is  attached.  The 
body  is  also  about  four  or  five  times  as  long  when  extended  as  when 
contracted.  At  average  extension  the  body  is  slightly  pointed  anteriorly 
and  the  acetabulum  is  but  little  more  than  half  the  distance  from  the 
anterior  to  the  posterior  end.  The  preacetabular  region  is  a  little  wider 
than  the  postacetabular,  and  the  postacetabular  tapers  slightly  toward 
its  truncated  posterior  extremity. 

In  mounted  specimens  the  average  length  of  the  body  is  0.4  mm., 
and  the  widest  diameter  of  the  preacetabular  region  averages  0.13  mm., 
while  that  of  the  pqstacetabular  is  0.09  mm.  The  width  of  the  tail  at  its 
base  varies  from  0.03  mm.  to  0.054  mm.,  depending  on  the  state  of 
contraction.  The  oral  sucker  is  lightly  elongate,  having  an  average 
length  of  0.05  mm.  and  a  width  of  0.045  mm.  The  acetabulum  is  slightly 
larger  and  circular,  averaging  0.054  mm.  in  diameter. 

The  structure  of  the  tail  of  Cercaria  megalura  (Figs.  30  and  32) 
is  interesting  on  account  of  its  remarkable  power  of  extension  and  the 
modification  of  its  posterior  end  for  attachment.  It  is  truncated  and 
there  is  an  invagination  at  its  tip.  Into  this  inpushing  open  a  clump  of 
from  15  to  20  unicellular  club-shaped  glands.  These  glands  have  an 
average  length  of  0.017  mm.  and  width  of  0.009  mm.,  and  contain  nuclei 
measuring  0.005  mm.  in  diameter.  It  is  probable  that  these  glands 
secrete  some  substance  which  makes  possible  the  adhesion  of  the  end 
of  the  tail.  The  tail  is  strongly  attached  at  its  base  and  except  near  the 
tip  appears  to  be  filled  with  vesicles,  which  are  stretched  out  when  the 
tail  is  extended  and  compressed  when  it  is  contracted.  Figure  32 
represents  a  cross  section  of  the  tail.  The  cuticula  is  very  thin,  the 
muscle  layers  reduced,  and  *the  great  bulk  of  the  tissue  is  made  up  of 
parenchymatous  cells,  the  nuclei  of  which  are  surrounded  by  small 
masses  of  protoplasm.  These  cells  are  connected  by  strands  of  proto- 
plasm and  the  large  intercellular  spaces  which  are  filled  with  clear  fluid 
give  the  appearance  of  vesicles.  It  is  the  looseness  of  the  tissue  of  the 
tail  which  makes  possible  its  remarkable  changes  in  shape. 

The  cystogenous  glands  of  Cercaria  megalura  are  very  highly  de- 
veloped and  when  the  cystogenous  material  is  still  present  in  them  they 
from  the  bulk  of  the  whole  body.  These  glands  (Fig.  33  cs)  are  large, 
elongate,  club-shaped  cells,  most  of  which  open  on  the  ventral  surface, 
a  few  only  opening  dorsally.  They  are  full  of  tiny,  rod-shaped,  cysto- 
genous granules.  The  cells  have  an  average  length  of  0.036  mm.  and  a 
width  of  0.012  mm.,  and  contain  at  about  their  centers  large  nuclei. 
They  are  present  all  thru  the  body  of  the  cercaria  from  the  posterior 
limit  of  the  oral  sucker  almost  to  the  posterior  extremity.  A  comparison 
of  cross  sections  of  two  cercariae  at  the  region  of  the  esophagus  shows 


481]  LARVAL   TREMATODES—CORT  35 

the  striking  changes  made  in  the  glands  and  all  the  tissues  by  the 
extrusion  of  the  cystogenous  material  (Figs.  33  and  34).  The  cysto- 
genous  glands  in  figure  34  have  become  much  reduced  in  size,  their  cyto- 
plasm is  only  slightly  granular,  and  even  their  nuclei  appear  to  have 
shrunk.  In  fact  they  are  distinguished  only  with  difficulty  from  the 
body  parenchyma.  The  extruded  material  forms  a  layer  about  0.0075 
mm.  in  thickness,  closely  adhering  to  the  animal.  After  the  extrusion 
of  the  cystogenous  material  the  cercaria  becomes  somewhat  shorter, 
broader,  and  thinner  than  before. 

The  mouth  of  Cercaria  megalura  is  subterminal  and  the  oral  cavity 
leads  into  a  narrow  prepharynx  about  the  length  of  the  oral  sucker  but 
varying  greatly  with  the  state  of  contraction.  The  pharynx  is  small, 
measuring  on  the  average  0.022  mm.  in  length  and  0.020  mm.  in  width, 
and  opens  by  a  narrow  passage  into  a  much  wider  portion  of  the  eso- 
phagus. This  soon  divides  into  the  narrow  intestinal  ceca  which  reach 
almost  to  the  posterior  end  of  the  body.  The  lining  of  the  enlarged 
portion  of  the  esophagus  and  the  intestinal  ceca  is  formed  by  flattened 
pavement  cells  with  flattened  nuclei,  which  are  illustrated  in  figures  33 
and  34.  Figures  29  and  30  both  show  the  relations  of  the  digestive 
system. 

At  the  posterior  end  of  the  body  is  found  the  pyriform  excretory 
vesicle  which  also  has  its  walls  formed  of  flattened  cells  with  flattened 
nuclei.  From  the  anterior  end  of  the  vesicle  pass  forward  two  longi- 
tudinal vessels  which  can  be  followed  to  the  region  of  the  pharynx 
(Fig.  30).  The  excretory  pore  was  not  distinguished.  In  Cercaria  dis- 
tomatosa  Looss  (1896:200)  described  the  excretory  system  as  having  a 
short  common  trunk  in  the  tail,  leading  from  the  bladder  and  opening 
to  the  outside  by  two  short  ducts.  Cary  in  his  material  of  Cercaria 
megalura  located  the  excretory  pore  as  dorsal  just  at  the  base  of  the 
tail,  and  found  no  extension  of  the  excretory  system  into  the  tail.  The 
most  careful  examination  of  the  anterior  part  of  the  tail  of  Cercaria 
megalura  in  living  specimens,  toto  mounts,  and  sections  showed  no  trace 
of  excretory  tubules. 

The  anlage  of  the  reproductive  organs  (Fig.  30  ra)  appears  as  a 
small  mass  of  nuclei  just  in  front  of  the  excretory  bladder;  from  this 
a  line  of  nuclei  extends  forward  to  join  another  small  nuclear  aggrega- 
tion just  in  front  of  the  acetabulum.  No  definite  outlines  of  organs 
could  be  determined.  The  mass  just  anterior  to  the  acetabulum  repre- 
sents to  the  ends  of  the  reproductive  ducts. 

In  the  literature  are  found  descriptions  of  only  two  cercariae  re- 
sembling Cercaria  megalura:  Cercaria  distomatosa  Sonsino  best  de- 
scribed by  Looss  (1896:197-204),  and  a  cercaria    from   the    Hawaiian 


36  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [482 

Islands  from  Melania  baldwini  Annecy  and  Melania  newcombii  Lea, 
described  briefly  by  Lutz  ( 1893 :327 ) .  Since  in  these  forms  there  are  no 
fixed  larval  organs  such  as  spines,  stylets,  etc.,  to  compare,  specific  dif- 
ferentiation is  difficult.  Cercaria  distomatosa,  however,  seems  to  show 
some  constant  differences  from  Cercaria  megalura.  In  Cercaria  disto- 
matosa the  rediae  reached  a  length  of  1.8  mm.,  had  a  pharynx  0.07  mm. 
long,  and  a  digestive  system  extending  only  to  the  posterior  locomotor 
appendages,  which  are  about  three-fourths  of  the  distance  from  the 
anterior  to  the  posterior  end.  Also  rediae  were  found  in  which  rediae 
were  developing.  In  Cercaria  megalura  as  noted  above  the  digestive 
system  was  more  voluminous  and  reached  almost  to  the  posterior  end  of 
the  body,  and  the  posterior  locomotor  appendages  were  further  back, 
being  about  six-sevenths  of  the  distance  from  the  anterior  to  the  posterior 
end.  Also  Looss'  measurements  for  Cercaria  distomatosa  are  slightly 
larger  than  those  for  Cercaria  megalura,  and  the  judge  from  his  figure 
and  description,  the  tail  is  considerably  smaller.  He  found  the  final  tub- 
ules of  the  excretory  system  in  the  tail,  which  were  not  found  in  my 
species.  The  Hawaiian  cercaria  described  by  Lutz  (1893:337)  seemed  to 
be  similar  to  Cercaria  megalura  and  Cercaria  distomatosa  in  having  a 
long  tail  by  which  it  can  become  attached,  in  the  general  relationship  of 
the  body  structures,  and  in  the  form  of  the  cyst.  A  more  detailed  com- 
parison is  impossible  on  account  of  the  meagerness  of  Lutz's  description. 
It  is  evident  that  these  three  forms  are  very  closely  related.  Cary's 
attempt  to  relate  Cercaria  megalura  to  Diplodiscus  temporatus  is  the 
only  suggestion  as  to  the  life-history  of  a  member  of  this  group,  but  as 
shown  above,  this  cannot  be  accepted. 


ECH1NOSTOME    CERCARIAE 

Cercariae  belonging  to  the  family  Echinostomidae  are  very  easily 
recognizable  on  account  of  their  structural  correspondence  to  the  adults. 
It  is  impossible  to  subdivide  them  into  smaller  natural  groups.  The 
following  may  be  given  as  a  brief  statement  of  characters  for  the 
Echinostome  cercariae. 

1.  Distome  cercariae  developed  in  rediae. 

2.  Rediae  have  collar,  birth-pore,  and  posterior  locomotor  append- 
ages. 

3.  Cercaria  with  digestive  system  consisting  of  prepharynx, 
pharynx,  long  esophagus,  and  intestinal-  ceca  reaching  the  posterior  end 
of  the  body. 

4.  Anterior  end  of  cercaria  with  collar  and  circle  of  spines. 


483]  LARVAL   TREMATODES—CORT  37 

5.  Excretory  system  opening  on  each  side  of  the  anterior  part  of 
the  tail ;  excretory  bladder  small,  crura  large,  reaching  to  oral  sucker. 

6.  Tail  powerful,  longer  than  body. 

In  the  material  used  in  this  study  were  two  echinostome  cercariae. 

The  first  species  was  found  in  several  specimens  of  Planorbis  trivol- 
vis, examined  during  November,  1913,  from  a  small  pond  near  Urbana, 
Illinois.  The  infection  consisted  of  rediae  containing  cercariae  in  the 
livers,  and  encysted  cercariae  in  the  body  cavities  of  the  snails.  Planor- 
bis trivolvis  is  then  able  to  serve  both  as  intermediate  and  secondary 
intermediate  host  for  this  species.  I  propose  for  this  cercaria  the  name 
Cercaria  trivolvis.  The  second  of  these  larvae  was  found  in  a  few  out 
of  thirty-six  specimens  of  Campeloma  subsolidum  from  Hartford,  Con- 
necticut. The  snails  in  this  case  I  regard  simply  as  the  secondary 
intermediate  host  since  no  rediae  were  found.  From  the  fact  that  the 
encysted  cercaria  had  a  pinkish  tinge  produced  by  pigment  granules 
in  the  postacetabular  region  the  name  Cercaria  rubra  is  proposed  for 
this  species. 

Cercaria  trivolvis  (Fig.  39)  completes  its  development  before  leav- 
ing the  redia.  Therefore  very  few  cercariae  were  found  free  in  the 
liver  of  the  host.  That  a  certain  time  is  spent  in  free  life  is  suggested 
by  the  fact  that  altho  free  swimming  cercariae  were  kept  under  observa- 
tion for  a  whole  day  none  were  seen  to  encyst.  There  seemed  to  be  no 
connection  between  infection  in  the  liver  and  cysts  in  the  body  cavity, 
since  altho  a  few  snails  had  both  types  of  infection  the  majority  had 
only  one. 

This  cercaria  (Fig.  39)  moved  actively  both  in  open  water  and  on 
a  substratum.  The  tail  was  powerful  and  extended  when  the  animal 
was  swimming  to  two  or  three  times  the  body  length.  For  the  swimming 
movement  the  cercaria  bent  ventrad  almost  double,  with  the  posterior 
half  of  the  body  almost  directly  dorsad  of  the  anterior.  The  tail  which 
extended  beyond  the  anterior  end  lashed  vigorously  and  propelled  the 
animal  rapidly.  When  a  cercaria  came  in  contact  with  a  surface  it 
took  hold  with  its  suckers  and  moved  actively  with  a  creeping  movement 
similar  to  that  already  described  for  Cercaria  megalura.  The  structure 
and  position  of  the  crown  of  spines  suggests  that  it  would  be  of  consider- 
able aid  to  the  animal  in  making  its  way  thru  connective  tissue. 

The  cysts  of  this  species  were  oval,  having  an  average  length  of 
0.16  mm.  and  width  of  0.15  mm.  The  cyst  wall  varied  in  thickness 
from  0.007  mm.  to  0.012  mm. 

Cercaria  trivolvis  (Fig.  39)  is  elongate,  pointed  anteriorly,  and  has 
its  greatest  width  in  the  region  of  the  acetabulum.     It  has  an  heart- 


38  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [484 

shaped  anterior  end  with  a  crown  of  thirty-seven  spines.  Well  extended 
toto  mounts  have  an  average  length  of  0.38  mm.  and  a  width  of  0.12 
mm.  The  tail  is  large  and  powerful  in  proportion  to  the  size  of  the 
body,  and  in  a  state  of  average  extension  has  a  length  of  0.50  mm. 
When  contracted  it  may  have  a  length  considerably  less  than  that  of 
the  body  and  when  the  eercaria  is  swimming  it  often  reaches  to  two  or 
three  times  that  length.  It  ends  in  a  sharp  point  and  at  the  tip  for 
about  0.05  mm.  to  0.06  mm.  it  is  narrow  and  is  composed  only  of  the 
muscle  layers,  lacking  the  parenchymatous  core. 

The  oral  sucker  is  almost  exactly  spherical  and  has  an  average 
diameter  of  0.043  mm.  The  acetabulum  is  a  little  larger,  being  0.049 
mm.  in  diameter,  and  is  situated  two-thirds  of  the  distance  from  the 
anterior  to  the  posterior  end. 

The  body  from  the  anterior  end  to  the  acetabulum  is  set  thickly 
with  rows  of  small  spines,  only  visible  under  the  highest  powers  of  the 
microscope. 

The  crown  contains  thirty-seven  spines  of  equal  size,  arranged  in 
two  alternate  rows,  broken  in  the  middle  of  the  ventral  surface.  They 
are  arranged  regularly  except  for  the  two  or  three  nearest  the  mid-line 
on  the  ventral  side,  which  point  in. 

The  body  from  the  oral  sucker  to  the  attachment  of  the  tail  is  filled 
with  cystogenous  glands.  They  are  unicellular  and  club-shaped  and  all 
open  on  the  dorsal  surface. 

The  oral  sucker  is  followed  by  a  typical  prepharynx  about  0.022 
mm.  in  length.  The  pharynx  is  round,  on  the  average  0.017  mm.  in 
diameter.  The  esophagus  and  intestinal  ceca  are  not  yet  functional, 
but  appear  merely  as  columns  of  granules  enclosed  in  membranes  and 
containing  irregular  spaces  representing  the  beginning  of  the  lumina. 
The  esophagus  is  long,  reaching  almost  to  the  acetabulum,  and  the  ceca 
reach  nearly  to  the  posterior  end  of  the  body. 

Cercaria  trivolvis  (Fig.  39)  has  the  typical  echinostome  excretory 
system.  The  portion  in  the  tail  was  made  out  with  great  difficulty  and 
is  apparently  not  functional.  A  vessel  passes  back  from  the  excretory 
bladder  for  one-fifth  or  one-sixth  of  the  length  of  the  tail  and  sends  out 
two  lateral  branches  which  open  to  the  outside.  Dorsally  at  the  base 
of  the  tail  is  the  adult  excretory  pore  which  apparently  at  this  stage 
gives  passage  to  the  outside  for  the  waste  material,  since  altho  the 
vesicle  kept  filling  and  emptying  the  vessels  of  the  tail  did  not  change 
their  caliber.  In  the  oldest  specimens  the  crura  are  large  and  distended 
from  the  bladder  to  the  region  of  the  pharynx  with  regularly-shaped, 
highly  refractive  granules.  For  the  most  part  these  are  round  or  oval, 
but  some  of  them  appear  to  be  compounded  of  from  two  to  four  of  the 


485]  LARVAL   TREMATODES—CORT  39 

round  ones.  At  the  region  of  the  pharynx  the  excretory  tubules  are 
much  smaller  and  curve  around  forming  a  characteristic  loop,  from 
which  a  small  vessel  on  each  side  could  be  traced  back  to  the  level  of  the 
acetabulum. 

The  nuclei  which  form  the  anlage  of  the  reproductive  organs  of 
Cercaria  trivolvis  are  not  yet  fully  divided  into  separate  masses.  In 
the  midline  of  the  body  just  in  front  of  the  excretory  bladder  and 
behind  the  acetabulum  is  an  elongate  mass  of  these  nuclei  from  which 
a  line  can  be  traced  forward  to  a  smaller  mass  in  front  of  the  acetabu- 
lum. The  posterior  mass  probably  develops  into  the  ovary  and  testes, 
and  the  anterior  mass  represents  the  ends  of  the  reproductive  ducts. 

Rediae  of  Cercaria  trivolvis  (Figs.  37  and  38)  were  present  in  the 
infected  snails  in  various  stages  of  development.  The  youngest  were 
unpigmented  but  in  the  older  ones  orange  colored  pigment  had  devel- 
oped in  the  outer  wall,  which  rendered  the  largest  rediae  almost  opaque. 

The  smallest  redia  found  had  a  length  of  0.30  mm.  and  a  width  of 
0.065  mm.  The  posterior  locomotor  appendages  were  0.22  mm.  from 
the  anterior  end  and  the  region  back  of  them  was  attenuated  and 
pointed.  The  oral  sucker  had  a  length  of  0.041  mm.  and  width  of  0.043 
mm.  and  the  intestine,  which  was  very  narrow  and  elongate,  reached  to 
the  posterior  locomotor  appendages.  No  germ  balls  were  present  in  the 
body  cavity  which  was  but  little  larger  than  the  intestine,  and  the  region 
of  the  germ  gland  was  not  visibly  differentiated. 

In  a  redia  (Fig.  38)  0.41  mm.  in  length  and  0.081  mm.  in  width 
the  posterior  locomotor  appendages  were  0.34  mm.  from  the  anterior 
end.  A  ridge  extended  around  the  body  like  a  welt  0.086  mm.  from 
the  anterior  end.  The  body  cavity  at  this  stage  had  become  well  devel- 
oped and  contained  germ  balls,  none  of  which  however  were  in  front  of 
the  posterior  extremity  of  the  intestine.  The  pharynx  had  a  length  of 
0.043  mm.  and  a  width  of  0.046  mm.  and  the  intestine,  which  was  wider 
than  the  earlier  stage  and  contained  dark  material,  reached  more  than 
one-half  the  body  length. 

The  great  majority  of  rediae  found  were  well  advanced  in  devel- 
opment and  contained  mature  cercariae  in  their  body  cavities  (Fig.  37). 
The  body  cavity  had  increased  in  size  extending  from  the  birth-pore 
almost  to  the  posterior  end,  and  into  the  posterior  locomotor  append- 
ages. In  each  redia  were  germ  balls  and  from  two  to  four  mature 
cercariae,  the  bodies  of  which  were  from  one-third  to  one-half  the  total 
length  of  the  redia.  In  a  mature  redia  0.81  mm.  in  length  and  0.15  mm. 
in  width,  the  posterior  locomotor  appendages  were  0.62  mm.  from  the 
anterior  end.  The  pharynx  was  0.041  mm.  long  and  0.043  mm.  wide 
and  the  intestine  which  was  somewhat  distended  with  food  material  and 


40  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [486 

pushed  ventrad  by  the  cercariae,  extended  for  more  than  one-third  of 
the  length  of  the  body. 

The  pharynx  is  about  the  same  size  in  the  youngest  and  the  oldest 
rediae,  and  the  actual  size  of  the  intestine  differs  but  little,  altho  its 
ratio  to  the  size  of  the  body  is  much  less  in  the  older  form.  The  birth- 
pore  is  located  on  the  dorsal  side  of  the  body  a  little  back  of  the  pharynx. 
In  the  oldest  rediae  the  anterior  collar  is  not  present  and  the  posterior 
locomotor  appendages  are  much  reduced. 

Cysts  of  Cercaria  rubra  (Fig.  41)  were  present  in  the  tissue  above 
the  gills  of  six  of  the  thirty-six  specimens  of  Campeloma  snhsolidum 
from  Hartford,  Connecticut.  The  cysts  were  large,  round,  thick-walled, 
and  of  very  uniform  size,  measuring  0.195  mm.  to  0.205  mm.  in  diame- 
ter. The  cyst  wall  was  transparent  and  had  a  thickness  of  0.016  mm. 
The  worm  almost  completely  filled  the  cyst  with  practically  the  whole 
dorsal  surface  against  the  wall,  and  the  posterior  end  overlapping  the 
anterior. 

Several  cysts  were  opened  and  the  worms  freed.  A  study  was 
made  of  these  while  living  but  none  were  preserved. 

The  living  cercariae  (Fig.  40)  were  on  the  average  0.50  mm.  in 
length  and  0.15  mm.  in  width  at  the  region  of  the  heart-shaped  anterior 
end.  They  tapered  slightly  posteriorly,  had  a  width  at  the  acetabulum 
of  0.13  mm.  and  the  end  was  bluntly  rounded. 

The  oral  sucker  of  Cercaria  rubra  was  a  most  exactly  round  having 
a  transverse  diameter  of  0.043  mm.  and  the  acetabulum  which  is  two- 
thirds  of  the  distance  from  the  anterior  to  the  posterior  end  was  larger, 
measuring  0.065  mm. 

The  collar  which  is  typical  of  the  Echinostomes  is  very  well  defined 
in  this  species  and  has  arranged  around  its  edge  in  two  alternating  rows, 
forty-three  spines,  which  vary  only  from  0.018  mm.  to  0.022  mm.  in 
length.  In  the  middle  of  the  ventral  surface  as  is  usual  there  is  a 
depression  and  a  break  in  the  rows  of  spines.  The  four  median  spines 
on  each  side  of  this  space  are  not  in  regular  line  with  the  others  and 
point  inward.  Besides  these  eight  there  are  seventeen  spines  in  the 
upper  row  and  eighteen  in  the  lower  row  (Figs.  40  and  42).  The 
surface  of  the  body  as  far  back  as  the  acetabulum  was  covered  thickly 
with  rows  of  spines  pointing  backward.  They  were  0.005  to  0.007  mm. 
in  length.  The  rows  were  0.008  mm.  apart  and  the  spines  were  set 
thickly  in  the  rows. 

The  digestive  system  (Fig.  40)  offered  nothing  peculiar.  The  pre- 
pharynx  had  a  length  about  equal  to  the  diameter  of  the  oral  sucker, 
and  the  pharynx  had  a  diameter  of  0.025  mm.    The  short  esophagus  and 


487]  LARVAL   TREMATODES—CORT  41 

intesinal  ceca  contained  the  granules  and  their  regular  spaces  described 
for  Cercaria  trivolvis. 

Only  the  main  branches  of  the  excretory  system  could  be  traced. 
The  crura  were  packed  very  full  of  concretions  and  the  bladder  was 
narrow. 

Since  this  species  was  only  studied  alive  the  anlage  of  the  repro- 
ductive system  could  not  be  distinguished. 

In  comparison  with  the  large  numbers  of  adult  Echinostomes  known 
the  descriptions  of  but  few  larval  forms  are  to  be  found  in  the  literature. 
Cercaria  rubra  differs  markedly  from  all  of  these  in  the  number  and 
arrangement  of  the  oral  spines.  Cercaria  trivolvis  on  the  other  hand 
agrees  very  closely  with  Cercaria  echinata  von  Siebold.  This  form  is 
not  sufficiently  described  to  make  a  detailed  comparison  possible.  Liihe 
(1909:188)  gives  to  it  thirty-seven  spines  of  equal  size  in  the  crown, 
altho  the  earlier  writers  counted  only  thirty-six.  The  intestine  of  the 
redia  is  shorter  in  Cercaria  echinata  than  in  Cercaria  trivolvis,  and  the 
size  of  the  oral  sucker  in  comparison  to  the  acetabulum  is  less  in  the 
former  than  in  the  latter  species. 

A  number  of  suggestions  have  been  made  in  regard  to  the  adults 
corresponding  to  the  European  species  of  echinostome  cercariae.  None 
of  these  proposed  relationships  have  been  proved  by  experiments.  Liihe 
(1909:65)  recently  made  the  following  statement  in  regard  to  this 
group: 

"von  den  bisher  mit  Namen  unterschiedenen  Cercarien  (samtlich 
aus  Susswasserschnecken)  ist  noch  keine  mit  volliger  Sicherheit  auf 
eine  bestimmte  Art  zu  beziehen." 

In  England  two  echinostome  cercariae  have  been  assigned  to  adults 
on  the  basis  of  morphological  comparisons.  The  cercariae  were  not 
given  special  names;  the  adults  are  Echinostomum  leptosomum  Creplin 
by  Lebour  (1907:447-451)  and  Echinostomum  secundum  Nicoll  by 
Nicoll  (1906:517-518). 

None  of  the  adult  echinostomes  from  North  America  agree  in  the 
number  and  arrangement  of  the  anterior  spines  with  the  two  cercariae 
described  above. 

The  following  form,  for  which  I  propose  the  name  Cercaria  reflexae, 
and  which  was  found  in,  some  of  the  specimens  of  Lymnaea  reflexo  from 
Chicago,  111.,  will  be  treated  as  an  appendix  to  the  Echinostome  cercariae. 
The  livers  of  the  snails  infected  with  Cercaria  reflexae  were  packed  with 
rediae  in  various  stages  of  development  of  which  the  greatest  numbers 
were  large  and  full  of  numerous  mature  cercariae.  Sections  of  the  liver 
showed  that  but  very  few  cercariae  were  free  in  its  tissues  but  large 


42  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [488 

numbers  were  freed  when  the  organ  was  removed,  from  the  snail.  Also  in 
the  body  cavities  of  numbers  of  the  same  snails  were  found  encysted  cer- 
cariae  of  the  same  species.  That  the  cercariae  were  continually  making 
their  way  out  and  encysting  in  new  snails  was  shown  by  the  fact,  that 
while  during  the  first  few  days  after  the  snails  came  into  the  laboratory 
only  a  few  contained  the  encysted  cercariae,  later  all  were  infected. 

The  movement  of  Cercaria  reflexae  both  in  open  water  and  in  a 
substratum  was  exactly  like  that  of  Cercaria  trivolvis. 

The  body  varied  greatly  in  size  and  shape.  When  contracted  for 
locomotion  it  was  nearly  as  wide  as  long.  A  fairly  well  extended 
mounted  specimen  (Fig.  43)  is  pointed  anteriorly,  widest  at  about  the 
level  of  the  acetabulum  and  narrower  at  the  posterior  end.  The  anterior 
end  does  not  suggest  the  echinostome  collar,  and  there  is  no  crown  of 
spines.  One  well  extended  specimen  had  a  length  of  0.46  mm.  and  a 
width  at  the  acetabulum  of  0.135  mm.,  with  a  thickness  of  a  little  more 
than  half  the  width. 

The  tail  at  average  extension  has  a  length  a  little  greater  than  the 
body,  and  an  average  width  at  its  base  of  0.05  mm.  to  0.06  mm.  The 
tail  is  provided  with  a  dorsal  and  a  ventral  fin-like  ridge  which  is  nar- 
row at  its  base  and  at  its  widest  part  equals  about  one-half  the  diameter 
of  the  tail. 

The  oral  sucker  has  a  diameter  of  0.046  mm.  and  the  acetabulum 
which  is  just  back  of  the  middle  of  the  body  is  0.06  mm.  in  width. 

The  surface  of  the  body  back  to  the  region  about  half  way  from  the 
acetabulum  to  the  posterior  end  is  covered  with  small  spines  arranged 
in  rows  and  set  closely  together. 

The  whole  body .  from  the  oral  sucker  to  the  posterior  extremity 
contains  large  unicellular  cystogenous  glands  which  open  dorsally,  and 
fill  the  bulk  of  the  body  toward  the  dorsal  side.  These  are  like  the 
cystogenous  glands  already  described  for  Cercaria  megalura  and  the 
amphistomes. 

The  oral  cavity  is  followed  by  a  short  prepharynx  and  a  small 
pharynx,  0.022  mm.  in  diameter.  The  esophagus  and  the  intestinal  ceca 
were  very  small  and  could  only  be  followed  in  sections.  The  esophagus 
reaches  almost  to  the  acetabulum  and  the  intestinal  ceca  to  the  posterior 
end. 

The  excretory  system  of  Cercaria  reflexae  (Fig.  43,  ex)  is  very 
much  like  that  of  Cercaria  trivolvis.  In  the  former  species  the  vessels 
from  the  bladder  to  the  tail  are  larger  and  serve  to  carry  away  the 
excretory  products,  for  when  the  excretory  bladder  became  contracted 
they  became  distended  but  soon  again  became  reduced.  The  excretory 
crura   too   are    smaller   than   in   the  other  species.    Opening   into   the 


489]  LARVAL   TREMATODES—CORT  43 

crura  could  be  traced  smaller  vessels  leading  from  the  posterior  end  of 
the  body,  and  forming  a  loop  at  the  region  of  the  pharynx  similar  to 
that  of  the  echinostomes. 

The  nervous  system  shows  very  prominently  both  in  living  and 
preserved  specimens  and  its  larger  branches  can  easily  be  traced.  It 
shows  nothing  peculiar.  From  the  large  masses  on  each  side  of  the 
prepharynx  two  branches  pass  out  to  the  oral  sucker.  Laterally  there 
are  two  branches  on  each  side  in  the  pharyngeal  region,  and  strands 
can  be  traced  almost  to  the  posterior  end  of  the  body. 

That  the  specimens  of  Cercaria  reflexae  which  came  under  my  ob- 
servation were  well  along  in  development  is  attested  by  the  condition  of 
the  reproductive  anlage  (Fig.  43).  It  is  divided  into  four  definite 
areas.  In  the  region  of  the  mid-line  just  in  front  of  the  excretory 
bladder  are  three  masses  of  nuclei,  one  in  front  of  the  other  and  close 
together.  The  posterior  two  which  are  smaller  than  the  other  probably 
represent  the  anlage  of  the  testes,  while  the  anterior  larger  mass  of 
nuclei  the  anlage  of  the  ovary  and  the  structures  surrounding  it.  Just 
in  front  of  the  acetabulum  is  a  mass  of  nuclei  representing  the  future 
ends  of  the  reproductive  ducts.  The  course  of  the  ducts  connecting 
these  areas  could  not  be  traced. 

The  rediae  of  Cercaria  reflexae  (Fig.  45)  were  present  in  different 
stages  of  development  in  the  infected  snails,  altho  there  was  a  great 
preponderance  of  the  fully  matured  forms.  The  smaller  rediae  were 
very  active  in  extending  and  contracting  their  bodies,  and  were  able 
to  make  progress  with  the  aid  of  their  posterior  locomotor  appendages. 
The  largest  forms  were  sluggish  and  reduced  to  mere  sacs  containing 
cercariae. 

The  smallest  rediae  have  much  the  same  structure  as  those  of 
Cercaria  trivolvis.  One  of  them  measured  0.28  mm.  in  length  and  0.054 
mm.  in  width  and  had  the  posterior  locomotor  appendages  0.22  mm. 
from  the  anterior  end.  The  anterior  ridge  or  collar  was  very  promi- 
nent, 0.022  mm.  from  the  anterior  end,  and  the  pharynx  had  a  diameter 
of  0.043  mm.  A  number  of  small  germ  balls  were  present  in  the  body 
cavity  and  the  intestine  extended  to  the  region  of  the  posterior  locomotor 
appendages. 

A  redia  somewhat  larger  than  the  one  just  described  is  shown  in 
figure  44.  In  this  form  the  cercariae  are  beginning  to  be  developed  into 
recognizable  form. 

The  nervous  system  was  quite  well  developed  in  the  young  rediae 
of  this  species.  In  one  very  immature  specimen  studied  while  alive 
two  large  nervous  masses  (Fig.  46,  n)  could  be  distinguished  on  each 
side  of  the  posterior  part  of  the  oral  sucker,  which  sent  two  branches 


44  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [490 

forward  and  two  backward  soon  to  become  lost  in  the  body  wall.  Very 
few  observations  on  the  nervous  system  of  the  redia  are  found  in  the 
literature.  Looss  (1896:199)  traced  the  nervous  system  of  the  redia  of 
Cercaria  distomatosa  about  as  far  as  it  is  followed  in  the  above  descrip- 
tion. He  speaks  of  this  as  the  best  developed  of  the  nervous  systems 
described  in  the  redia  up  to  that  time.  The  degree  of  development  of 
the  nervous  system  as  he  suggests  is  correlated  with  the  degree  of 
mobility  of  the  redia. 

In  the  largest  of  the  rediae  (Fig.  45)  almost  all  the  germ  balls 
have  developed  into  mature  cercariae,  of  which  from  twenty  to  thirty  are 
present.  The  body  cavity  is  much  enlarged,  the  cercariae  having  pushed 
clear  into  the  posterior  tip,  into  the  posterior  locomotor  appendages 
and  up  around  the  oral  sucker.  The  rediae  are  mere  shells,  the  anterior 
collar  having  been  obliterated  and  the  posterior  locomotor  appendages 
much  reduced.  The  length  of  one  of  the  largest  rediae  found  was  2.26 
mm.,  the  width  0.30  mm.  and  the  posterior  locomotor  appendages  were 
1.65  mm.  from  the  anterior  end.  The  oral  sucker  is  very  little  different 
in  size  from  that  in  the  younger  specimens,  its  diameter  being  0.056 
mm.  The  intestine  was  long  and  slender  and  reached  to  a  point  1.15 
mm.  from  the  anterior  end. 

That  Cercaria  reflexae  is  closely  related  to  the  echinostomes  is  shown 
by  a  comparison  with  Cercaria  trivolvis.  The  rediae  of  the  two  species 
have  much  the  same  general  structure.  In  fact  it  would  be  practically 
impossible  to  distinguish  between  very  young  rediae  of  the  two  forms. 
Further,  the  locomotion  and  general  body  structures  of  the  two  cercariae 
are  similar.  The  arrangement  of  the  anlage  of  the  reproductive  organs 
of  the  two  forms  is  much*  alike.  That  Cercaria  reflexae  is  simply  an 
immature  echinostome  cercaria  in  which  the  crown  of  spines  has  not 
developed  is  improbable  from  the  fact  that  its  other  structures  are 
those  of  a  well  advanced  cercaria,  the  anlagen  of  the  reproductive 
organs  especially  being  considerably  differentiated.  No  record  has  been 
found  of  any  species  either  cercaria  or  adult  which  agrees  with  Cercaria 
reflexae. 

MICROCERCOUS   CERCARIAE 

Among  thirty-six  specimens  of  Campeloma  subsolidum  from  Hart- 
ford, Connecticut,  four  were  found  to  be  infected  with  rediae  and  very 
short  tailed  cercariae.  The  infection  was  in  the  tissues  of  the  body 
above  and  at  the  bases  of  the  gills.  This  form,  which  has  a  very  short 
triangular  tail,  I  propose  to  name  Cercaria  trigonura. 

The  cercariae  were  free  in  the  tissues  of  the  snail,  were  numerous, 
and  were  all  in  the  same  stage  of  development.    When  freed,  they  ex- 


491]  LARVAL   TREMATODES—CORT  45 

tended  and  contracted  their  bodies  rapidly,  the  preacetabular  region 
being  the  most  active,  but  they  were  unable  to  swim.  The  tail,  bent 
ventrad  and  pushing  against  the  substratum,  aided  somewhat  in  loco- 
motion, and  at  times  the  oral  sucker  was  used  for  attachment.  By  this 
peculiar  method  the  animal  was  able  to  make  a  little  progress  with  a 
great  deal  of  effort.  Leuckart  (1886:86)  notes  the  same  type  of  move- 
ment for  Cercaria  limacis  Moulinie,  a  stumpy  tailed  cercaria  from  lAmax, 
cineria. 

"Ausser  Stande  zu  schwimmen,  benutzen  diese  Wurmer  den  fast 
hertzformigen  Schwanzanhang  beim  Kriechen  als  Nachschieber. " 
Some  of  these  cercariae  were  kept  alive  and  active  in  tap-water  for 
three  or  four  days. 

Cercaria  trigonura  (Figs.  48,  50)  has  an  elongate,  cylindrical  body, 
averaging  in  mounted  specimens  0.24  mm.  in  length  and  0.06  mm.  in 
width.  The  oral  sucker  measures  on  the  average  0.049  mm.  in  length, 
and  0.039  mm.  in  width  and  the  acetabulum  which  is  slightly  back  of  the 
center  of  the  body  is  smaller,  being  0.04  mm.  in  diameter.  The  cuticula 
is  thin  and  at  the  anterior  end  beset  with  tiny  spines,  which  are  numer- 
ous over  the  oral  sucker,  thin  out  posteriad  and  disappear  entirely  back 
of  the  pharynx. 

The  tail  of  Cercaria  trigonura  is  short,  easily  detached,  grooved 
ventrally,  and  has  the  extremity  bluntly  pointed  (Figs.  48,  50).  It 
has  an  average  length  of  0.052  mm.  and  a  width  of  0.024  mm.  Under 
the  cuticula  is  a  thin  layer  of  circular  muscles  which  are  supplemented 
ventrally  by  a  number  of  stronger  longitudinal  fibers  that  extend  from 
the  base  to  the  tip.  The  size  of  these  strands  and  the  fact  that  they 
have  no  opposing  muscles  on  the  dorsal  side  would  account  for  the 
fact  that  the  tail  is  usually  bent  ventrad.  The  bulk  of  the  tail  is  formed 
of  loose  parenchymatous  tissue  consisting  of  scattered  nuclei,  connecting 
protoplasmic  strands  and  good  sized  vesicles. 

On  the  ventral  surface  of  Cercaria  trigonura  just  at  the  base  of 
the  tail  is  a  slit-like  opening,  which  extends  forward  a  short  distance 
and  dorsad  reaches  up  into  the  body.  Opening  into  this  cavity  are 
large  numbers  of  unicellular  glands  which  stain  very  heavily  with 
haematoxylin.  Figure  48  pg  shows  the  relation  of  this  posterior  glandu- 
lar structure.  It  is  interesting  that  the  shape  and  position  of  the  tail 
give  it  the  appearance  of  a  short  trough  ready  to  carry  off  the  secre- 
tions of  this  gland.  The  position  and  structure  of  the  posterior  gland 
suggest  that  it  may  function  for  adhesion.  No  activity  which  suggests 
such  a  function  has  been  observed  and  for  none  of  the  other  stumpy 
tailed  cercariae  has  the  description  of  a  posterior  glandular  structure 
been  found. 


46  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [492 

Set  in  the  dorsal  part  of  the  oral  sucker  and  protruding  for  about 
one-fifth  of  its  length  is  a  cephalic  spine  or  stylet  (Figs.  47,  49).  The 
spine  has  a  length  of  0.018  mm.  and  a  width  of  0.004  mm.,  is  sharply 
pointed  and  slightly  thickened  about  two-thirds  of  the  distance  from 
the  base  to  the  tip.    Ventrally  no  thickening  is  present. 

The  cephalic  or  stylet  glands  (Fig.  48,  sg)  fill  the  space 
dorsad  and  anteriad  to  the  acetabulum  and  extend  forward  to  just  back 
of  the  nervous  system.  The  individual  glands  are  small,  averaging 
0.024  mm.  in  length  and  0.012  mm.  in  width,  finely  granular  and  con- 
tain small  nuclei.  Their  ducts  pass  forward  dorsally  over  the  oral 
sucker  and  open  around  the  cephalic  spine  in  the  anterior  pit.  The 
numbers  were  so  large  that  no  accurate  count  could  be  made.  They 
form  a  single  mass  unbroken  in  the  median  line. 

The  mouth  is  subterminal  and  back  of  the  oral  sucker  is  a  small 
pharynx,  0.012  mm.  in  length  by  0.01  mm.  in  width.  The  prepharynx 
is  short  and  the  esophagus  and  intestinal  ceca  are  entirely  undeveloped. 

The  excretory  pore  of  Cercaria  trigonura  opens  dorsally  at  the 
posterior  extremity  of  the  body  just  at  the  base  of  the  tail.  Leading 
up  to  it  is  a  narrow  tube  near  the  dorsal  surface  which  expands  into 
the  large  bicornuate  excretory  vesicle,  the  horns  of  which  reach  on  each 
side  of  the  acetabulum.  Vessels  could  be  traced  along  the  sides  of  the 
body  from  the  tips  of  the  horns  of  the  vesicle  up  to  the  pharynx. 
Figures  48  and  50  give  the  relations  of  the  pore  vesicle  and  vessels. 
The  vesicle  is  lined  with  a  thick  layer  of  granular,  cuboidal,  epithelial 
cells  with  large  nuclei  (Fig.  54,  ex).  An  excretory  vesicle  with  thick- 
ened walls  of  the  character  .just  described  has  been  found  in  the  other 
so-called  stumpy-tailed  forms. 

The  anlage  of  the  reproductive  organs  shows  as  a  mass  of  nuclei 
dorsal  to  the  posterior  part  of  the  acetabulum  (Fig.  48,  ra). 

Along  with  the  cercariae  in  the  tissues  of  Campeloma  subsolidum 
were  large  numbers  of  rediae  of  different  sizes.  They  were  found  in 
every  snail  infected  with  Cercaria  trigonura  and  were  not  found  in  any 
instance  where  this  species  was  not  present.  The  only  rediae  that 
showed  any  activity  were  very  small  immature  forms  which  were  present 
in  considerable  numbers  and  in  the  same  stage  of  development.  They 
were  active,  extending  and  contracting  and  twisting  and  turning  in 
all  directions.  Part  of  them  were  sharply  pointed  and  the  others  bluntly 
rounded  posteriorly.  The  first  type  (Fig.  51)  was  the  most  common 
and  the  structure  of  these  will  be  described  in  some  detail.  The  avera- 
age  of  the  mounted  specimens  of  the  small  rediae  is  0.019  mm.  in  length 
and  0.04  mm.  in  width.     The  pharynx  is  small,  0.03  mm.  in  width 


493]  LARVAL   TREMATODES—CORT  47 

and  set  back  slightly  from  the  anterior  end  so  that  the  anterior  part 
of  the  oral  cavity  is  in  front  of  it.  The  intestine  is  narrow  and  elongate 
reaching  nearly  to  the  end  of  the  body  and  almost  filling  the  small 
body  cavity.  Inside  of  the  cuticula  and  muscle  layers  the  wall  is  made 
up  of  embryonic  nuclei  with  poorly  defined  cell  boundaries.  There 
are  present  no  developing  embryos  and  the  germ  gland  is  not  clearly 
differentiated. 

Very  few  rediae  were  found  showing  intermediate  stages  between 
the  small  type  just  described  and  the  largest  forms.  The  one  shown 
in  Figure  53  with  only  five  germ  balls  of  any  size  in  its  body  cavity  is 
0.43  mm.  in  length  and  0.09  mm.  in  diameter.  The  pharynx  has  the 
same  width  as  length,  0.043  mm.,  and  the  pouch  shaped  intestine,  which 
is  0.11  mm.  long  and  0.038  mm.  at  its  greatest  width,  extends  to  a  point 
only  about  one-third  of  the  distance  from  the  anterior  to  the  posterior 
end.  The  body  cavity  is  large  and  contains  oval  germ  balls,  the  largest 
of  which  is  0.076  mm.  long  and  0.06  mm.  wide.  The  wall  of  the  cavity 
is  about  two  cells  thick  and  the  germ  gland  is  clearly  defined  con- 
taining both  single  germ  cells  and  balls  of  several  cells.  The  pharynx 
in  this  form  is  also  set  back  from  the  anterior  end. 

The  largest  rediae  (Fig.  52)  are  all  rounded  at  their  posterior 
extremities  and  their  body  cavities  are  very  large  with  thin  walls.  The 
germ  gland  is  much  reduced.  At  one  side  near  the  anterior  end  is  located 
the  birth  pore  with  protruding  lips.  The  rediae  vary  in  size  up  to  the 
largest  measured  which  is  0.73  mm.  in  length.  The  largest  ones  have 
from  25  to  30  germ  balls  of  about  the  same  size  in  their  body  cavities. 
The  intestine  is  smaller  in  proportion  to  the  size  of  the  body  but  of 
about  the  same  absolute  size  as  in  the  smaller  forms,  and  the  oral  sucker 
is  very  clearly  set  back  from  the  anterior  end.  The  outer  layers  at  the 
anterior  part  are  wrinkled  so  that  the  appearance  is  given  of  a  series 
of  horizontal  folds  from  the  anterior  tip  back  to  the  middle  of  the  body. 

Besides  the  large  rediae  with  the  body  cavity  full  of  germ  balls 
there  are  present  numbers  of  large  rediae  with  few  or  no  germ  balls  and 
much  constricted  and  twisted. 

That  these  rediae  are  the  nurse  generations  of  Cercaria  trigonura 
is  very  probable  from  their  being  found  in  every  instance  with  that 
form.  All  the  most  immature  rediae  are  about  the  same  size  and  appear 
to  belong  to  the  same  brood,  yet  there  is  no  evidence  as  to  their  origin. 
Neither  is  there  any  evidence  where  the  cercariae  free  in  the  tissues 
come  from,  for  in  none  of  the  rediae  can  cercariae  be  distinguished. 
The  old  rediae  shells  may  have  been  the  nurses  of  these  cercariae  and 
rediae  but  even  when  a  few  embryos  are  found  in  these,  they  are  merely 
large  little  differentiated  germ  balls.     It  is  possible  that  in  these  snails 


48  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [494 

were  represented  several  infections  and  that  the  approach  of  winter, 
for  the  examinations  were  made  in  November,  may  have  arrested  the 
development  of  the  second  brood. 

The  only  possible  group  in  Liihe's  (1909)  classification  of  the  cer- 
cariae  where  Cercaria  trigonura  might  fit  is  with  the  Microcercous  cer- 
cariae.  An  analysis  of  the  forms  coming  under  this  category  shows  that 
they  cannot  form  a  natural  group.  Dollfuss  (1914)  in  the  preliminary 
account  of  his  work  on  this  group  separates  from  it  the  Cotylocercous 
cercariae  which  he  considers  to  be  a  natural  subdivision. 

He  gives  the  following  characters  for  the  Cotylocercous  cercariae. 

1.  Cercariae  developing  in  simple  sporocysts  parasitic  in  marine 
gastropods. 

2.     Oral  sucker  with  stylet ;  stylet  glands  fill  a  large  part  of  the 
anterior  region  of  the  body. 

3.  Bladder  large  not  bifurcate  occupying  almost  all  the  posterior 
region  of  the  body;  wall  formed  by  a  single  layer  of  large  granular 
cells,  which  have  the  appearance  of  glands. 

4.  Tail  very  short,  wide  at  least  at  its  base,  consisting  of  a  cup  with 
thick  walls  of  large  cells,  which  functions  as  a  sucker. 

Dollfuss  is  in  doubt  in  regard  to  the  specific  distinctness  of  some 
of  the  members  of  this  group.  He  therefore  designates  them  together 
provisionally  under  the  name  Cercaria  pachycerca  Diesing  "sensu  lato 
et  var."  Under  this  designation  he  includes  Cercaria  brachyura  Lespes, 
Cercaria  cotylura  Pagenstecker,  and  several  new  forms.  Besides  these 
are  included  in  the  Cotylocercous  cercariae  as  undoubtedly  distinct 
species  Cercaria  linearis  Lespes  and  Cercaria  buccini  sp.  inq.  Lebour. 
Dollfuss  states  in  regard  to  this  group  that  the  cercariae  are  so  alike 
in  structure  that  the  adults  must  be  closely  related. 

Aside  from  the  cercaria  of  Catoptroides  macrocotyle  Liihe  (Phyllo- 
distomum  folium  Ssinitzin,)  and  metacercariae,  which  have  been  related 
to  the  Microcercous  cercariae  without  any  good  reason,  there  are  left 
after  the  separation  of  the  Cotylocercous  cercariae,  five  more  or  less  well 
known  forms. 

1.  Cercaria  limacis  (MoulhnV  (1856:83,  163-164)  from  sporo- 
cysts in  the  terrestrial  molluses  Arion  rufus  L.  and  Limax  cinereus  0.  P. 
Miiller. 

2.  Cercaria  micrura  de  Filippi  (1857:5-7)  (larva  of  Sphaerosto- 
mum  bramae  (0.  F.  Miiller)  from  sporocysts  in  the  freshwater  snail 
Bithynia  tentaculata. 

3.  Cercaria  myzura  Pagenstecher  (1881:25-26)  from  rediae  in  the 
fresh-water  mollucs  Neritina  (Theodocia)  fluvialvis  L. 


495]  LARVAL   TREMATODES—CORT  49 

4.  Cercaria  trigonura  mihi  from  rediae  in  the  fresh-water  snail 
Campeloma  subsolidum. 

5.  Cercaria  columbellae  Pagenstecher  (1862:305-306)  from  rediae 
in  the  marine  mollucs  Columbella  rustica  L. 

Of  the  two  of  the  above  cercariae  developed  in  sporocysts  only 
Cercaria  micrura  agrees  in  structure  with  the  characters  given  for  the 
Cotylocercous  cercariae.  The  only  reason  for  not  including  it  in  this 
group  is  apparently  that  it  is  a  fresh-water  form.  Just  why  it  should 
be  excluded  from  this  group  for  this  reason  when  it  agrees  with  them 
in  structure  is  not  clear. 

Cercaria  myzura  Pagenstecher,  Cercaria  columbellae  Pagenstecher, 
and  Cercaria  trigonura  mihi  are  the  three  stumpy-tailed  forms  which 
develop  from  rediae.  Two  are  from  fresh-water  and  one  is  marine. 
Both  of  Pagenstecher 's  forms  are  so  insufficiently  known  that  but  little 
structural  comparison  is  possible.  Cercaria  myzura  and  Cercaria  colum- 
bellae both  have  the  truncated  tail  like  Cotylocercous  group. 

Cercaria  trigonura  is  unique  among  the  stumpy-tailed  forms  in 
having  a  large  posterior  gland  opening  at  the  base  of  the  tail  and  a 
bicornuate  excretory  vesicle.  It  differs  from  all  except  Moulinie's  (1856  :- 
83,  163-164)  Cercaria  limacis  in  having  a  blunted  tail,  which  is  not  mod- 
ified as  a  sucker. 

FURCOCERCOUS     CERCARIAE 

Sporocysts  containing  small  forked-tailed  cercariae  with  eye-spots, 
were  found  in  five  out  of  thirty-eight  specimens  of  Lymnaea  reflexa  from 
a  small  pond  in  the  suburbs  of  Chicago,  Illinois.  I  propose  to  name  this 
form  Cercaria  douthitti. 

The  livers  of  the  snails  infected  with  Cercaria  douthitti  were  filled 
with  a  tangled  mass  of  elongate,  cylindrical  sporocysts.  The  walls  of 
the  sporocysts  were  so  thin  and  in  such  close  contact  with  the  liver 
lobes  (Fig.  63,  sw),  that  it  was  impossible  to  free  individual  sporo- 
cysts and  to  follow  them  to  any  length.  They  were  irregular  tubes  of 
varying  caliber,  had  no  free,  mobile,  club-shaped  ends  sticking  out,  and 
were  filled  with  large  numbers  of  embryos  in  various  stages  of  develop- 
ment. (Fig.  64).  The  walls  of  the  sporocysts  were  made  up  of  a  very 
thin  fibrous  layer  on  the  inside  of  which  were  scattered  nuclei  (Fig  63). 

None  of  the  cercariae  were  found  outside  of  the  sporocysts  in  the 
snail,  but  when  the  liver  was  dissected  large  numbers  worked  their  way 
out.  Their  progress  from  place  to  place  was  quite  erratic  altho  they 
moved  their  bodies  and  tails  vigorously.  When  in  locomotion  the  body 
and  tail  were  both  somewhat  contracted  and  both  moved  back  and  forth. 


50  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [496 

The  movement  of  the  tail  was  not  a  lashing  as  in  many  forms,  but  a 
vibration  in  which  the  middle  region  was  the  most  active.  The  cerearia 
kept  catching  hold  with  its  acetabulum  and  extending  its  anterior  end, 
but  was  not  able  to  take  hold  with  the  oral  sucker,  which  even  in  the 
older  specimens  was  not  fully  developed.  Sometimes  when  a  cerearia 
under  observation  was  pressed  lightly  with  a  cover-glass  it  would  catch 
hold  with  the  ventral  sucker  and  the  vibrations  of  the  tail  would  cause 
it  to  swing  round  and  round  on  the  sucker  as  pivot. 

Cerearia  douthitti  is  a  very  small  form,  cylindrical  in  cross  section, 
slightly  wider  at  the  center  and  tapering  from  the  acetabulum  to  the 
posterior  end  to  a  width  equal  to  about  that  of  the  tail.  The  body  has 
an  average  length  in  well  extended  mounted  specimens  of  0.19  mm.  and 
a  width  of  0.067  mm.  The  tail  is  bifid  and  even  when  contracted  is 
equal  to  about  one  and  one-half  the  length  of  the  body.  The  lobes  form 
less  than  one-third  of  its  length  and  are  definitely  constricted  from  the 
main  portion,  making  the  tail  appear  as  if  jointed  (Fig.  55).  The 
main  stem  of  the  tail  has  an  average  length  of  0.22  mm.  and  a  width 
of  0.025  mm.,  while  the  lobes  have  about  half  that  width  and  an  average 
length  of  0.089  mm.  When  the  cerearia  is  alive  the  tail  has  a  range 
of  variation  from  less  than  once  to  about  twice  the  length  of  the  body. 
Underneath  the  thin  cuticula  and  the  muscle  layers  of  the  tail  is  a  layer 
of  unicellular  club-shaped  glands ( ?),  which  lie  close  together  and  have 
their  ducts  extending  forward  to  open  to  the  outside  along  the  sides. 
These  glands  were  only  seen  in  the  living  specimens.  The  central 
core  of  the  tail  is  composed  of  parenchymatous  tissue,  thru  which  flows 
the  caudal  branch  of  the  excretory  system.  The  base  of  the  tail  fits  into 
a  depression  at  the  posterior  end  of  the  bodv,  which  is  open  ventrally 
(Fig.  57). 

The  oral  sucker  of  Cerearia  douthitti  is  proportionally  large,  meas- 
uring on  the  average  0.057  mm.  in  length  and  0.045  mm.  in  width.  It 
is  a  mass  of  embryonic  cells  which  are  separated  from  the  surrounding 
tissue  by  a  fibrous  sheath,  except  that  the  ducts  of  the  cephalic  glands 
pass  thru  to  open  at  the  anterior  tip.  There  is  no  differentiation  into 
muscle  fibers  and  no  mouth  or  oral  cavity  is  marked.  Cerearia  ocellata, 
a  European  forked-tailed  form  which  corresponds  in  structure  very 
closely  to  Cerearia  douthitti  is  described  as  having  a  definite  mouth 
opening,  by  La  Valette  St.  George  (1855:22-23).  He  also  gives  measure- 
ments much  smaller  for  the  oral  suckers  than  those  of  Cerearia  douthitti, 
making  it  less  than  half  as  large  as  the  acetabulum  (0.013  mm.  to  0.033 
mm.)  Luhe  (1909:206)  questions  his  measurements,  and  since  in  La 
Valette  St.  George 's  drawing  that  region  is  not  clear,  it  may  well  be  that 
he  did  not  grasp  the  true  proportions  of  the  oral  sucker,  and  that  it  is  as 


497]  LARVAL   TREMATODES—CORT  51 

large  in  his  form  as  in  Cercaria  douthitti.  The  acetabulum  is  fully  func- 
tional as  noted  above.  It  is  very  small,  on  the  average  0.025  mm.  in  dia- 
meter, situated  just  back  of  the  middle  of  the  body. 

The  eye-spots  are  in  front  of  the  middle  of  the  body  and  located 
nearer  the  dorsal  side.  The  pigment  masses  which  compose  them  are 
in  the  shape  of  concavo-convex  discs,  0.007  mm.  in  diameter,  placed  so 
that  the  concave  surfaces  are  toward  the  sides  of  the  body.  Fitting  into 
the  concave  side  of  each  is  a  small  lens.  In  the  region  of  the  eyes  and 
lying  just  ventrad  and  behind  them  is  the  central  nervous  system.  A 
cross  section  of  the  cercaria  in  the  region  of  the  eye-spots  brings  out 
these  relations  clearly  (Fig.  56,  e). 

The  region  back  of  the  center  of  the  body  of  Cercaria  douthitti 
is  almost  completely  filled  with  large  unicellular  cephalic  glands,  the 
usual  number  of  which  is  eight.  The  most  anterior  reach  to  the  middle 
of  the  body  and  from  all,  large  ducts  extend  forward  in  two  groups. 
These  two  groups  of  ducts  pass  thru  the  sheath  of  the  oral  sucker  pos- 
teriorly and  traverse  this  organ  to  open  at  the  anterior  tip  of  the  body 
(Fig.  59,  60,  61,  62).  No  cephalic  spine  is  present  altho  these  glands 
appear  to  be  analogous  to  stylet  glands.  Cercaria  ocellata  La  Valette  St. 
George  (1855:22-23)  is  the  only  other  furcocercous  cercaria  in  which 
such  glands  are  described.  The  glands  are  flasked-shaped  and  have  a 
length  varying  from  0.04  mm  to  0.05  mm.  and  a  width  of  from  0.025 
to  0.03  mm.  The  thickness  is  about  equal  to  the  width.  A  traverse 
section  (Fig.  58)  thru  the  acetabular  region  of  Cercaria  douthitti  shows 
how  much  of  the  body  space  is  taken  up  by  the  cephalic  glands. 

At  the  posterior  end  of  the  body  of  Cercaria  douthitti  is  a  small 
excretory  vesicle  from  which  two  crura  could  be  traced  forward  only  as 
far  as  the  acetabulum.  Backward  from  the  bladder  two  small  vessels 
pass  into  the  tail  (Fig.  57,  ex).  These  soon  unite  into  the  central  caudal 
vessel  which  divides  to  run  down  the  lobes  and  opens  at  their  tips  (Fig. 
55).    No  concretions  of  any  kind  were  present  in  the  excretory  system. 

Just  in  front  of  the  excretory  bladder  and  wedged  between  the  tips 
of  the  cephalic  glands,  is  a  small  mass  of  nuclei  the  anlagen  of  the  repro- 
ductive organs. 

The  furcocercous  or  forked-tailed  cercariae  are  very  imperfectly 
known.  The  anatomy  of  only  a  few  of  the  known  forms  is  at  all  well 
worked  out  and  the  life-history  of  no  one  of  them  has  been  determined. 
At  least  a  dozen  species  have  been  reported  as  distinct,  some  of  which, 
however,  have  been  described  very  briefly  in  the  older  accounts.  Suffi- 
cient evidence  is  not  available  to  justify  any  conclusion  as  to  the  natural 
or  artificial  character  of  this  group. 


52  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [498 

Only  one  of  the  forked-tailed  cercariae,  Cercaria  ocellata  La  Valette 
St.  George  corresponds  at  all  closely  in  structure  to  Cercaria  douthitti. 
The  structure  of  this  form  has  been  fairly  well  worked  out  by  La  Valette 
St.  George  (1855:22-23)  and  Moulinie  (1856:172-173).  Cercaria  ocellata 
agrees  with  Cercaria  douthitti  in  the  large  unicellular  glands  of  the  pos- 
terior body  region,  in  the  presence  of  eyespots,  in  the  length  and  jointed 
character  of  the  tail,  and  in  fact  that  the  forked  portion  is  only  one-third 
of  its  total  length.  The  ratio  in  size  of  the  suckers  may  agree  as  stated 
above. 

Cercaria  ocellata  differs  in  several  particulars  from  Cercaria  douth- 
itti. The  former  is  almost  twice  as  large,  is  found  in  a  different  host, 
in  a  different  continent,  and  has  narrow  fin-like  extensions  on  the  divided 
lobes  of  the  tail. 

No  suggestion  can  be  made  as  to  the  life-history  of  Cercaria  douth- 
itti. Its  structure  is  such  as  not  even  to  suggest  to  what  family  of  dis- 
tomes  the  adult  belongs.  In  fact  hardly  a  suggestion  has  been  made  in 
regard  to  the  life-histories  of  the  forked-tailed  cercariae  and  no  experi- 
ments that  I  can  find  have  been  carried  on  to  trace  their  development. 
Certainly  further  studies  are  needed  on  their  structure  and  develop- 
ment. 

XIPHIDIOCERCARIAE 

Luhe  (1909:189)  defines  Diesing's  (1855)  group  of  the  Xiphidio- 
cercariae  or  stylet  cercariae  as  follows : 

Slender-tailed  distome  cercariae  with  a  boring  spine  on  the  rounded 
anterior  end.  Eyes  lacking  p  develop  in  sporocysts ;  encystment  in  a  sec- 
ondary intermediate  host. 

Since  this  group  is  formed  on  likenesses  in  but  few  larval  char- 
acters it  can  be  considered  only  within  wide  limits  as  expressing  rela- 
tionship. On  account  of  their  small  size  and  also  since  many  of  them 
are  known  only  from  the  older  accounts  many  of  the  forms  of  this  group 
are  very  insufficiently  described.  Five  new  forms  are  added  to  this 
group  by  the  study  of  my  material.  In  the  following  account  the  new 
American  forms  will  be  compared  with  the  most  closely  related  of  the 
already  known  species,  and  where  it  is  possible  an  attempt  will  be  made 
to  fit  them  into  natural  groups. 

Two  of  these  forms  with  the  related  European  species  agree  so 
closely  in  structure  that  a  new  group,  the  Polyadenous  cercariae  will 
be  formed  for  them. 


499]  LARVAL   TREMATODES—CORT  53 

POLYADENOUS      CERCARIAE 

In  eighteen  per  cent.,  of  170  specimens  of  Planorbis  trivolvis  col- 
lected from  the  drainage  ditch  north-east  of  Urbana,  Illinois,  the  livers 
were  filled  with  elongate,  cylindrical  sporocysts  very  much  twisted 
together.  The  sporocysts  were  not  branching  but  it  was  very  diflficut 
to  trace  out  the  individual  sacs.  When  this  could  be  done  they  were 
found  to  be  of  about  uniform  caliber  and  various  lengths  (Fig.  66). 
Two  that  were  measured  were  1.48  mm.  and  1.9  mm.  in  length  and 
varied  in  diameter  from  0.13  mm.  to  0.17  mm.  The  walls  were  thin 
and  contained  flecks  of  orange  pigment,  which  were  very  dense  in  the 
oldest  specimens.  Many  of  the  sporocysts  contained  large  numbers  of 
actively  moving  cercariae  which  would  escape  and  swim  about  freely 
when  the  liver  was  teased  apart.  The  wall  of  the  sporocyst  was  com- 
posed of  a  layer  of  pavement  epithelium  with  flattened  nuclei.  No 
thickenings  were  found  in  the  wall  and  no  traces  of  germ  gland,  altho 
small  germ  balls  were  free  in  the  cavity.  Cercariae  at  all  stages  of  devel- 
opment were  found  in  the  sporocysts  (Fig.  65).  I  propose  to  call  this 
species  Cercaria  isocotylea  from  the  fact  that  the  acetabulum  and  the 
oral  sucker  are  very  nearly  equal  in  size. 

Cercaria  isocotylea  (Fig.  68)  is  oval  elongate,  slightly  pointed 
anteriorly,  and  of  uniform  width  from  the  region  of  the  pharynx  back 
to  the  acetabulum.  The  length  and  the  width  varied  with  the  contrac- 
tion state  within  rather  wide  limits.  From  the  measurements  of  mounted 
specimens  of  moderate  contraction,  the  length  averages  0.17  mm.  and 
the  width  0.06  mm.  The  cross  section  is  oval  and  the  thickness  a  little 
greater  than  half  the  width.  The  tail  is  small  in  proportion  to  the  size 
of  the  body  and  set  in  a  groove  on  the  ventral  side  of  the  posterior  end. 
When  contracted  it  may  be  less  than  one-half  the  body  length,  but  when 
the  cercariae  is  swimming  it  may  be  extended  to  greater  than  that  length. 
Under  ordinary  circumstances  it  has  an  average  length  of  0.01  mm. 
and  a  width  at  its  base  of  0.02  mm. 

Cercaria  isocotylea  moved  actively  both  in  open  water  and  on  a 
substratum.  When  swimming  it  turned  so  that  the  ventral  side  was 
up,  the  body  was  contracted  and  bent  slightly  ventrad.  The  tail  became 
much  extended  and  lashed  rapidly  backward  and  forward.  It  did  not 
however  have  the  power  of  moving  the  animal  definitely  in  one  direction 
for  any  length  of  time,  and  locomotion  was  very  erratic.  Whenever 
while  swimming  the  cercaria  came  in  contact  with  a  surface,  the  tail 
ceased  its  lashing  and  the  body  began  to  stretch  and  reach  around  until 
the  oral  sucker  could  obtain  a  hold.  Then  the  animal  would  creep  along 
with  the  aid  of  its  suckers.     Sometimes  after  the  cercariae  had  come 


54  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [500 

in  contact  with  a  surface  and  extended  its  body,  the  tail  resumed  its 
iashings  and  swimming  was  started  again  with  the  body  extended.  This 
was  simply  preliminary  to  the  contraction  of  the  body  and  the  resump- 
tion of  the  usual  swimming  position. 

The  oral  sucker  of  Cercaria  isocotylea  (Fig.  68,  os)  has  an  average 
length  in  mounted  sections  of  0.04  mm.  and  a  width  of  0.037  mm.,  while 
the  acetabulum  which  is  spherical  in  the  living  animal,  is  about  two- 
thirds  of  the  distance  from  the  anterior  to  the  posterior  end  and  has  a 
diameter  of  0.036  mm. 

Set  in  the  dorsal  wall  of  the  oral  sucker  is  a  large  sylet  (Fig.  67) 
which  is  sharp  pointed  and  has  a  thickening  two-thirds  of  the  distance 
from  its  base  to  its  tip.  It  is  flattened  ventrally,  has  a  length  of  from 
0.028  mm.  to  0.030  mm.  and  a  width  at  its  base  one-sixth  of  its  length. 

The  whole  surface  of  the  body  back  to  the  anterior  margin  of  the 
acetabulum  is  thickly  set  with  tiny  cuticular  spines  which  are  0.003 
mm.  to  0.004  mm.  in  length.  Back  of  this  region  the  cuticula  is  entirely 
smooth. 

On  each  side  of  the  stylet  open  the  ducts  of  the  stylet  glands,  which 
form  a  group  on  each  side.  The  glands  are  elongate,  sac-shaped,  uni- 
cellular and  faintly  granular,  with  a  length  of  from  0.02  to  0.025  mm. 
and  a  width  of  from  0.011  to  0.013  mm.  They  form  two  clumps  of  from 
six  to  eight  in  a  clump,  in  the  region  just  in  front  and  to  each  side  of 
the  oral  sucker. 

In  this  stage  of  development  of  the  cercariae  no  cystogenous  glands 
are  present. 

Except  for  the  mouth,  oral  cavity,  very  short  prepharynx,  and 
pharynx  the  digestive  system  of  Cercaria  isocotylea  is  undeveloped.  The 
mouth  is  very  small  having  a  transverse  diameter  of  0.012  mm.  and  the 
round  pharynx  is  0.016  mm.  in  width. 

The  excretory  pore  opens  on  the  dorsal  side  just  at  the  base  of  the 
tail.  The  bladder  is  bicornuate  consisting  of  a  median  part  and  two 
lateral  horns  which  reach  on  each  side  up  to  the  middle  of  the  acetabulum. 
From  the  anterior  and  posterior  regions  on  each  side  can  be  traced  small 
vessels  which  unite  into  a  short  common  duct  to  open  at  the  tips  of  the 
horns.    (Fig.  68,  ex) 

The  anlage  of  the  reproductive  organs  is  not  divided  into  definite 
parts.  It  consists  of  a  mass  of  small  nuclei  which  lies  dorsad  and  just 
in  front  of  the  anterior  margin  of  the  acetabulum.  This  connects  with 
a  larger  mass  which  is  dorsal  to  the  posterior  part  of  the  acetabulum  by 
a  broad  band  running  around  dorsal  to  the  left  margin  of  the  sucker. 

Among  the  specimens  of  Lymnaea  reflexa  from  Chicago  was  one 
which  contained  a  number  of  elongated  unpigmented  sporocysts,  which 


501]  LARVAL   TREMATODES—CORT  55 

were  found  in  the  liver  with  the  rediae  of  Cercaria  reflexae.  These 
sporocysts  were  much  like  those  of  Cercaria  isocotylea,  and  were 
so  thin  walled  and  so  closely  interwoven  with  the  lobes  of  the  liver  that 
none  were  isolated  for  accurate  measurements.  Altho  large  numbers 
of  the  cercariae  in  the  sporocysts  were  mature,  few  were  free  in  the  liver 
of  the  host.  When,  however,  the  liver  was  taken  out  of  the  snail  many 
were  freed  and  swam  actively  about.  I  propose  the  name  Cercaria  poly- 
adena  for  this  species  from  the  fact  that  the  body  of  the  cercaria  contains 
such  large  numbers  of  gland  cells. 

The  position  of  Cercaria  polyadena  in  swimming  was  similar  to  that 
of  Cercaria  isocotylea.  The  tail  was,  however,  somewhat  stronger  than 
in  the  latter  species  and  the  animal  was  able  to  move  forward  definitely 
and  fairly  rapidly.  Whenever  the  cercaria  came  in  contact  with  a  sur- 
face, it  immediately  settled  down,  took  hold  with  its  suckers,  and  crept 
along. 

The  tail  was  very  easily  detached  from  the  body  and  would  con- 
tinue swimming  for  some  time  with  a  wriggling  motion.  Whenever  an 
actively  detached  tail  came  in  contact  with  the  substratum,  it  ceased 
wriggling  and  alternately  extended  and  contracted  as  if  it  were  still 
attached  to  a  living  cercaria.  One  of  the  detached  tails  kept  up  active 
movement  for  over  fifteen  minutes  and  then  was  stopped  by  the  drying 
up  of  the  water  around  it. 

Small  thin  walled  cyst  containing  tailless  individuals  of  Cercaria 
polyadena  were  found  scattered  in  with  the  material  preserved  for  study 
(Fig.  71).  It  is  probable  that  these  cysts  were  formed  after  the  liver 
was  removed  from  the  snail,  and  that  Lymnaea  reflexa  is  not  the  sec- 
ondary intermediate  host  of  this  species.  The  formation  of  the  cyst 
was  not  observed  but  that  the  glands  were  ready  for  secretion  is  shown 
by  the  fact  that  the  extrusion  of  the  cystogenous  material  was  observed 
in  one  individual  that  was  flattened  under  a  cover  slip.  The  cysts 
were  round  and  varied  in  diameter  from  0.15  mm.  to  0.16  mm.,  and 
the  transparent  cyst  wall  varied  in  thickness  from  0.005  to  0.007  mm. 

Cercaria  polyadena  (Fig.  70)  is  very  variable  in  shape,  living  speci- 
mens changing  from  0.12  mm.  when  contracted  to  0.30  mm.  at  greatest 
extension.  When  most  contracted  the  tail  may  be  less  than  one-half 
the  body  length,  but  when  the  animal  was  swimming  it  reaches  to 
0.30  mm.  The  average  measurements  of  five  mounted  individuals  in 
about  the  state  of  contraction  of  the  figure,  give  the  length  of  0.18 
mm.,  the  width  of  0.07  mm.,  the  length  of  the  tail  0.12  mm.  and  its 
width  0.017  mm.  The  tail  is  attached  in  a  groove  on  the  ventral  sur- 
face of  the  posterior  end,  and  altho  it  is  small  for  the  size  of  the  body 
it  is  relatively  larger  than  in  Cercaria  isocotylea. 


56  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [502 

The  oral  sucker  has  an  average  diameter  in  mounted  specimens 
of  0.040  mm.  and  the  spherical  acetabulum  which  is  three-fifths  of  the 
distance  from  the  anterior  to  the  posterior  end  is  0.025  mm.  wide. 

In  the  dorsal  wall  of  the  oral  sucker  is  set  a  stylet  like  that  of  Cer- 
caria isocotylea.  It  is  0.028  to  0.03  mm.  in  length  and  thickened  as  in 
the  other  species. 

The  whole  surface  of  the  body  is  set  with  tiny  spines  contained 
entirely  within  the  cuticula  and  not  set  in  rows;  these  thin  out  some- 
what in  the  postacetabular  region. 

Two  groups  of  voluminous  ducts  pass  up  dorsad  to  the  oral  sucker 
from  the  stylet  glands,  which  fill  most  of  the  space  from  the  acetabulum 
to  the  pharynx.  The  diameter  of  one  of  these  ducts  is  from  0.004  mm. 
to  0.005  mm.  The  stylet  glands  are  divided  into  two  groups  of  from 
ten  to  twelve  in  a  group,  and  the  individual  glands  vary  in  length  from 
0.017  to  0.022  mm.,  and  in  width  from  0.010  mm.  to  0.014  mm. 

Cystogenous  glands  are  present  both  on  the  dorsal  and  ventral  sides 
of  the  body  from  the  pharynx  to  the  posterior  end. 

In  Cercaria  polyadena  the  oral  cavity  is  followed  by  a  short  pre- 
pharynx  0.015  mm.  in  diameter.  No  traces  were  seen  either  of  the 
esophagus  or  the  intestinal  ceca. 

The  excretory  bladder  is  bicornuate  and  the  excretory  pore  is 
located  dorsally  at  the  base  of  the  tail.  The  two  horns  of  the  vesicle 
extend  up  to  the  posterior  lateral  margins  of  the  acetabulum,  and  each 
receives  a  small  vessel.  This  receives  a  long  vessel  from  the  region  of 
the  pharynx  and  a  short  vessel  from  the  posterior  end.  The  lining 
of  the  vesicle  is  formed  of  a  layer  of  slightly  flattened  cuboidal  epi- 
thelial cells  with  prominent  nuclei  and  well  defined  cell  boundaries. 

The  anlage  of  the  reproductive  organs  is  not  differentiated  into  its 
individual  parts.  It  is  represented  by  an  elongate  s-shaped  mass  of  nuclei 
lying  dorsad  to  the  acetabulum  and  extending  backward  beyond  its 
posterior  margin. 

Cercaria  polyadena  and  Cercaria  isocotylea  are  very  much  alike  and 
form  the  nucleus  of  a  group  the  members  of  which  present  such  uni- 
formity of  characters  that  they  must  be  considered  to  be  related.  The 
name  Polyadenous  cercariae  may  then  be  proposed  as  the  name  of  a 
natural  group  the  members  of  which  correspond  closely  to  Cercaria  poly- 
adena. 

The  following  are  the  characters  of  the  Polyadena  cercariae : 

1.  Development  in  gastropods  in  elongate  sac-shaped  sporocysts. 

2.  Tail  slender  and  less  than  the  body  length  except  when  very 
much  extended. 


503]  LARVAL   TREMATODES—CORT  57 

3.  Acetabulum  back  of  the  middle  of  the  body  and  smaller  than 
the  oral  sucker. 

4.  Stylet  about  0.030  mm.  in  length,  six  times  as  long  as  broad, 
and  with  a  thickening  one-third  of  the  distance  from  the  point  to  the 
base. 

5.  Stylet  glands,  six  or  more  on  each  side  between  the  acetabulum 
and  the  pharynx. 

6.  Excretory  bladder  bicornuate. 

7.  Very  short  prepharynx  and  small  pharynx  present.  Esophagus 
when  developed  short  to  of  medium  length.  Intestinal  ceca  (when  pres- 
ent reaching  to  posterior  end  of  body). 

Two  European  fresh-water  cercariae,  Cercaria  limnaeae  ovatae 
von  Linstow  and  Cercaria  secunda  Ssinitzin,  without  doubt  belong  to 
this  group.  They  both  agree  in  all  known  particulars  with  the  char- 
acterization given  above.  No  mention  is  made,  however,  in  von  Lin- 
stow's  (1884)  account  of  stylet  glands. 

There  are  definite  specific  differences  between  the  four  forms  which 
constitute  the  Polyadenous  cercariae.  Of  the  two  American  forms 
Cercaria  polyadena  has  a  larger  body  and  tail,  a  smaller  oral  sucker,  and 
a  larger  number  of  stylet  glands.  Cercaria  limnaeae  ovatae  is  the  largest 
of  the  group,  has  much  larger  suckers  than  any  of  the  others,  and  is 
developed  in  larger  sporocysts.  The  closest  correspondence  is  between 
Cercaria  polyadena  and  Cercaria  secunda  Ssinitzin.  These  two  species 
are  certainly  very  closely  related.  Cercaria  secunda  is,  however,  larger 
in  size  and  has  slightly  larger  suckers  and  fewer  stylet  glands  than 
Cercaria  polyadena. 

Some  suggestion  can  be  made  in  regard  to  the  type  of  adults  into 
which  the  cercaria  of  this  group  develop.  Cercaria  limnaeae  ovatae 
has  been  assigned  to  Opisthioglyphe  rastellus  (Liihe,  1909:108)  and 
Ssinitzin  (1905)  suggests  that  Cercaria  secunda  may  be  the  larva  of  a 
Plagiorchis  species.  It  would  seem  probable  from  the  above  facts  and 
the  structure  of  the  excretory  and  digestive  systems  that  the  Polyadenous 
cercariae  belong  in  Liihe 's  subfamily  Plagiorchiinae  which  contains 
Opisthioglyple  as  well  as  Plagiorchis.  The  further  development  of  the 
two  American  forms  is  entirely  unkown. 

CERCARIAE  ORNATAE 

In  5  per  cent,  of  the  specimens  of  Physa  gyrina  from  Rockford, 
Illinois,  the  body  contained  a  tangled  mass  of  elongated,  orange  pig- 
mented sporocysts.    The  tubes  did  not  branch,  they  were  of  varying 


58  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [504 

caliber  and  club-shaped  ends  protruded  from  the  mass.  They  were  very 
much  twisted  together  and  none  of  the  individual  sacs  were  disentangled 
without  breaking.  The  jutting  ends  moved  slightly,  swaying  backward 
and  forward. 

Thruout  their  whole  length  the  sporocysts  (Fig.  77)  were  stuffed 
with  cercariae  of  different  ages,  and  mature  forms  in  large  numbers 
wormed  out  of  the  broken  places  in  the  sporocysts.  The  swimming 
movement  of  the  cercaria  offered  nothing  peculiar  and  since  the  oral 
sucker  did  not  appear  to  function,  creeping  was  not  very  effective. 
The  name  Cercaria  hemilophura  is  proposed  from  the  fact  that  a  fin- 
Jike  projection  extends  for  half  the  length  of  the  tail. 

Cercaria  hemilophura  (Fig.  76)  is  oval  elongate  in  shape  and  widest 
at  about  the  middle.  The  average  length  in  well  extended  mounted 
specimens  is  0.38  mm.  and  the  width  0.14  mm.,  with  a  thickness  of  about 
one-half  the  width. 

The  tail  at  average  extension  is  about  the  length  of  the  body, 
0.36  mm.  being  the  average  in  toto  mounts  and  with  a  width  of  0.048 
mm.,  but  it  can  be  extended  to  almost  twice  that  length.  Along  the 
ventral  surface  of  the  posterior  half  of  the  tail  extends  a  fin-like  projec- 
tion, which  at  its  widest  is  about  half  the  width  of  the  tail. 

The  oral  sucker  has  a  length  of  0.065  mm.  and  the  acetabulum, 
which  is  just  back  of  the  middle  of  the  body,  has  a  diameter  of  0.049  mm. 

The  stylet  (Fig.  75)  is  small,  tapers  regularly  to  a  point  and  has 
no  thickened  region.  It  measures  0.020  mm.  in  length  and  0.005  mm. 
in  width  at  its  base. 

The  whole  surface  of  the  body  is  covered  with  very  small  cuticular 
spines  pointing  backward,  which  are  very  dense  in  the  preacetabular 
region  but  thin  out  slightly  posteriorly.  They  are  contained  entirely 
within  the  thickness  of  the  cuticula  and  have  a  length  of  from  0.0055 
mm.  to  0.0065  mm. 

The  whole  body  contains  large  numbers  of  small  cystogenous  glands 
filling  almost  all  the  available  space. 

Stylet  glands  could  not  be  distinguished. 

The  digestive  system  of  Cercaria  hemilophura  is  very  clearly  differ- 
entiated. The  oral  cavity  is  followed  by  a  very  short  prepharynx  and 
a  good  sized  pharynx  0.033  mm.  in  diameter.  From  the  pharynx  a  large 
esophagus  reaches  back  almost  to  the  acetabulum.  The  esophagus  is  thin 
walled  but  the  intestinal  ceca  are  lined  with  cuboidal  cells  which  at  this 
stage  fill  most  of  the  lumina. 

It  was  possible  in  many  cases  to  trace  the  branches  of  the  excretory 
system  to  the  flame  cells.  The  bladder  is  club-shaped,  extending  about 
three-fourths  of  the  distance  from  the  posterior  end  to  the  acetabulum, 


505]  LARVAL   TREMATODES—CORT  59 

and  widens  slightly  at  its  anterior  end.  Into  it  flow  two  vessels  from 
the  region  of  the  oral  sucker  on  each  side,  which  are  met  by  vessels  from 
the  posterior  end  at  the  region  of  the  acetabulum.  Small  branches  lead 
from  the  flame  cells  and  connect  with  these  branches.  Figure  76  shows 
the  excretory  system  of  Cercaria  hemilophura. 

The  anlagen  of  the  reproductive  organs  are  represented  by  a  two 
lobed  mass  of  nuclei  dorsal  to  the  acetabulum. 

According  to  Liihe  's  classification  of  the  Xiphidiocercariae  Cercaria 
hemilophura  would  belong  with  the  group  Cercariae  ornatae.  He  gives 
the  following  definition  for  this  group : 

' '  Distome  Cercarien  mit  Bohrstachel,  deren  schlanker  Ruderschwanz 
einen  Flossensaum  besitzt. ' ' 

He  included  in  this  group  Cercaria  ornata  La  Valette  and  Cercaria 
prima  Ssinitzin.  This  is  certainly  not  a  natural  subdivision,  since  the 
three  forms  are  very  different  in  other  structures.  The  presence  of 
such  a  character  as  the  "Flossensaum"  hardly  forms  the  basis  for  a 
natural  group.  Since  it  has  been  developed  also  in  such  widely  different 
groups  as  the  monostomes  and  the  echinostomes.  At  the  present  state 
of  our  knowledge  it  seems  impossible  to  relate  Cercaria  hemilophura 
to  any  natural  group.  Neither  is  there  any  suggestion  as  to  the  further 
development  of  this  species. 

MICROCOTYLOUS     CERCARIAE 

The  tissue  above  the  gills  in  three  out  of  thirty-six  specimens  of 
Campeloma  subsolidum  from  Hartford,  Conn.,  was  heavily  infected  with 
oval  thin-walled  sporocysts.  The  sporocysts  had  granular,  somewhat 
opaque  walls,  varied  in  shape  from  almost  round  to  elongate  oval,  and 
contained  small  cercariae  in  different  stages  of  development.  None  of 
the  cercariae  were  fully  mature  and  none  were  found  free  in  the  tissues 
of  the  host.  There  was  little  movement  of  the  cercariae  either  within 
the  sporocysts  or  when  freed.  I  propose  the  name  Cercaria  leptacantha 
for  this  species  on  account  of  the  small  size  of  the  stylet. 

The  sporocysts  (Fig.  81)  varied  from  0.26  mm.  to  0.41  mm.  in  length 
and  from  0.15  mm.  to  0.26  mm.  in  width. 

The  body  of  Cercaria  leptacantha  (Fig.  80)  is  oval  elongate  and 
almost  circular  in  cross-section.  The  average  measurements  of  five  well 
extended  mounted  specimens  are  0.12  mm.  in  length  and  0.063  mm. 
in  width.  The  tail  is  not  fully  developed  still  remaining  as  a  direct 
continuation  of  the  body  and  having  little  power  of  movement.  It  is 
less  than  the  length  of  the  body  and  slender,  averaging  0.081  mm.  in 
length  and  0.016  mm.  at  its  greatest  width. 


60  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [506 

In  none  of  the  individuals  studied  were  the  suckers  fully  developed 
or  functional.  The  oral  sucker  shows  a  beginning  of  a  mouth  cavity 
but  the  acetabulum  is  merely  a  rounded  off  mass  of  embryonic  nuclei. 
In  mounted  specimens  the  oral  sucker  averages  0.027  mm.  in  diameter 
and  the  acetabulum,  which  is  back  of  the  middle  of  the  body,  has  a 
width  of  0.024  mm. 

On  the  surface  of  living  specimens  of  Cercaria  leptacantha  were 
scattered  highly  refractive  round  globules  of  different  sizes,  which 
appeared  like  water  or  oil  droplets.  These  bodies,  which  resemble  the 
concretions  in  the  excretory  systems  of  certain  cercariae,  were  so  prom- 
inent that  they  could  be  seen  thru  the  walls  of  the  sporocysts,  but  dis- 
appeared in  the  preservation  of  the  material. 

The  small  characteristic  stylet  (Fig.  79)  is  0.011  mm.  to  0.13  mm. 
in  length,  and  0.0025  mm.  in  thickness  at  its  base. 

Two  kinds  of  glands  were  present  in  Cercaria  leptacantha.  The 
first  type  consists  of  small  irregular  shaped  bodies  with  granular  con- 
tents at  the  anterior  lateral  margins  of  the  acetabulum.  The  other 
type  are  almost  clear  globlet  shaped  stylet  glands,  four  on  each  side 
arranged  along  the  body  lateral  to  the  acetabulum,  with  ducts  from 
their  outer  margins  leading  up  to  the  cephalic  spine.  No  ducts  were 
found  for  the  first  type  of  gland,  but  from  their  granular  contents  they 
may  be  cystogenous  in  character. 

The  digestive  system  of  Cercaria  leptacantha  is  represented  only 
by  a  short  prepharynx  and  a  small  pharynx  0.09  mm.  in  diameter. 

Of  the  excretory  system  only  the  elongate,  club-shaped  bladder  can 
be  distinguished. 

The  anlage  of  the  reproductive  organs  is  represented  merely  by  a 
large  mass  of  small  nuclei  dorsal  and  posterior  to  the  acetabulum. 

Cercaria  leptacantha  belongs  to  a  group  of  very  small  cercariae 
which  Luhe  (1909:196)  calls  Cercariae  Microcotylae.  It  is  possible  that 
they  form  a  natural  group.  They  are,  however,  so  insufficiently  known 
that  no  final  judgment  can  be  passed  on  their  relationships.  At  present 
it  seems  best  to  follow  Luhe  in  considering  them  a  provisional  group, 
with  Cercaria  microcotyla  Filippi  as  the  type  and  the  following  char- 
acteristics. 

1.  Developed  in  gastropods  in  round  or  oval  sporocysts  which  are 
seldom  more  than  twice  as  long  as  wide. 

2.  Cercariae  under  0.2  mm.  in  length. 

3.  Acetabulum  back  of  the  middle  of  the  body  and  smaller  than 
the  oral  sucker. 


507]  LARVAL   TREMATODES—CORT  61 

4.  Stylet  glands  not  more  than  four  on  each  side  and  arranged  in 
rows  on  each  side  of  the  acetabulum. 

5.  Digestive  system  undeveloped  except  for  a  short  prepharynx 
and  a  small  pharynx. 

Three  European  and  three  Egyptian  cercariae  are  sufficiently  known 
to  be  included  with  any  certainty  in  this  group.  The  European  forms 
are  Cercaria  pugnax  La  Valette,  Cercaria  microcotyla  Filippi,  and  Cer- 
caria  siibulo  Pagenstecher  (for  description  of  these  forms  see  Liihe,  1909  :- 
196-198),  and  the  Egyptian  forms  are  Cercaria  celluosa  sp.  inq.,  Cercaria 
pusilla  sp.  inq.,  and  Cercaria  exigua  sp.  inq.  all  described  by  Looss 
(1896:227-232).  Insufficiently  known  forms  which  from  their  small  size 
and  the  shape  of  the  sporocysts  may  belong  to  this  group  are  Cercaria 
chlorotica  Diesing,  Cercaria  alba  Ercolani,  and  Cercaria  punctum  Erco- 
lani.  Cercaria  parva  Ercolani  in  which  the  oral  sucker  is  smaller  than 
the  acetabulum  agrees  in  its  other  characters  with  the  members  of  this 
group. 

The  Microcotylous  cercariae  are  best  distinguished  from  each  other 
by  the  size  and  shape  of  their  stylets.  Cercaria  leptacantha  agrees  most 
closely  with  the  Egyptian  species  Cercaria  exigua.  It  is  larger  than  this 
species  however,  the  suckers  differ  in  size  and  the  ratio  of  size,  and  the 
stylets  differ  in  size  and  shape. 

Only  one  suggestion  is  found  in  regard  to  the  adults  of  this  group. 
Looss  (1896 :232)  considers  that  the  three  cercariae  of  this  type  described 
by  him  may  belong  to  some  small  distomes  found  in  Egypt  in  the  intes- 
tines of  chameleons  and  lizards.  He  offers  no  particular  grounds  for 
this  hypothesis. 

Last  will  be  considered  a  form  of  the  Xiphidiocercariae  which 
seems  to  be  different  from  all  forms  previously  described.  The  livers 
of  three  out  of  ninety-one  specimens  of  Physa  anatina  from  Manhattan, 
Kansas,  were  infected  with  sausage-shaped  sporocysts  (Fig.  72)  which 
contained  cercariae  in  different  stages  of  development.  I  propose  for  this 
species  the  name  Cercaria  brevicaeca  from  the  fact  that  the  intestinal 
ceca  are  very  short. 

Cercaria  brevicaeca  moved  clumsily  and  irregularly  while  swimming 
and  did  not  creep  by  aid  of  its  suckers.  The  infection  was  very  slight 
and  all  the  observations  were  made  from  living  specimens.  None  of  the 
cercariae  were  observed  to  live  more  than  two  or  three  hours  after  removal 
from  the  snail. 


62  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [508 

Cercaria  brevicaeca  (Fig.  74)  was  elongate  oval  in  shape  and  its  tail, 
which  was  very  easily  lost,  had  about  the  same  length  as  the  body. 
At  average  extension  the  body  had  a  length  of  about  0.30  ram.  and  a 
width  of  0.14  mm.  The  tail  did  not  change  its  shape  very  greatly  and 
ranged  in  length  only  from  0.22  mm.  to  0.38  mm.  with  a  width  at  its 
base  of  0.038  mm.  It  was  attached  in  a  groove  in  the  ventral  side  of 
the  posterior  end  and  when  contracted  had  a  tendency  to  curl  at  its 
tip. 

The  oral  sucker  of  Cercaria  brevicaeca  had  an  average  diameter 
of  0.082  mm.  and  the  acetabulum,  which  is  just  back  of  the  middle  of 
the  body,  was  slightly  larger,  0.087  mm. 

The  stylet  (Fig.  73)  had  a  length  of  0.018  mm.  and  was  slightly 
thickened  0.007  mm.  from  its  point. 

The  body  back  to  the  acetabulum  was  covered  thickly  with  rows 
of  very  tiny  spines. 

In  the  region  between  the  acetabulum  and  the  pharynx  were  two 
clumps  of  from  ten  to  twelve  stylet  glands,  varying  in  length  from 
0.026  mm.  to  0.035  mm.  and  in  width  from  0.018  mm.  to  0.025  mm. 
The  ducts  from  these  glands  united  into  two  groups  one  on  each  side 
which  passed  dorsad  of  the  oral  sucker  to  open  beside  the  stylet. 

Almost  every  bit  of  available  space  behind  the  pharynx  was  filled 
with  cystogenous  glands,  which  from  the  surface  appeared  as  round, 
granular  bodies  0.014  mm.  to  0.017  mm.  in  diameter. 

The  oral  cavity  was  followed  by  a  short  prepharynx  and  a  small 
pharynx,  0.030  mm.  in  diameter.  The  short,  narrow  esophagus  divided 
just  in  front  of  the  acetabulum  into  short  intestinal  ceca  which  did  not 
reach  beyond  the  acetabulum.  The  lumina  of  the  ceca  showed  only 
as  irregular,  elongate  spaces  in  the  granular  contents. 

Of  the  excretory  system  only  the  peculiarly  shaped  vesicle  could 
be  made  out.  This  was  composed  of  a  pyriform  median  portion,  and 
two  more  narrow  lateral  parts  which  almost  completely  surrounded  the 
acetabulum.    The  pore  opened  dorsad  at  the  base  of  the  tail. 

No  cercariae  were  found  in  the  literature  closely  corresponding  with 
Cercaria  brevicaeca.  Especially  unique  is  the  shape  of  the  excretory 
vesicle. 

THE   CLASSIFICATION   OP    THE   CERCARIAE 

At  the  present  state  of  our  knowledge  it  is  impossible  to  fit  most 
of  the  cercariae  into  the  general  trematode  classification,  for,  except 
in  those  forms  like  the  echinostomes  or  the  amphistomes,  where  the  larvae 
are  much  like  the  adults,  or  in  a  group  like  the  family  Gorgoderinidae, 
where  the  life  histories  of  several  species  have  been  worked  out,  little 


509]  LARVAL   TREMATODES—CORT  63 

is  definitely  known  of  the  relation  of  the  cercariae  types  to  the  adults. 
Therefore  for  convenience  it  has  seemed  advisable  to  build  up  a  tentative 
classification  of  the  cercariae,  treating  them  almost  as  if  they  were  an 
independent  class  of  the  animal  kingdom.  Of  necessity  such  a  classifica- 
tion must  be  based  pretty  largely  on  superficial  characters.  As  our 
knowledge  increases  wherever  possible  natural  groups  must  be  substituted 
for  the  artificial,  and  as  more  and  more  larvae  are  connected  with  the 
adults,  the  classification  of  the  cercariae  will  gradually  be  merged  with 
that  of  the  adults. 

In  order  to  understand  clearly  the  classification  that  has  been  made 
for  larval  trematodes,  a  careful  analysis  of  the  characters  used  for  com- 
parison must  be  made.  Cercarial  characters  can  be  roughly  divided 
into  two  main  groups:  (1)  adult  characters,  and  (2)  larval  characters. 
By  adult  characters  of  a  cercaria  are  meant  those  which  foreshadow 
adult  structure.  It  is  by  the  use  of  these  characters  as  a  basis  that 
the  greatest  progress  in  natural  classification  can  be  made,  since  the  more 
the  adult  characters  are  developed,  the  more  will  the  cercariae  resemble 
the  adults,  as  in  amphistome  and  echinostome  larvae.  For  example  the 
digestive  and  the  excretory  systems  of  the  cercariae  of  these  groups  are 
much  like  those  of  the  adult.  Sometimes  in  the  larva  definite  specific 
peculiarities  of  the  adult  can  be  distinguished  in  detail.  Thus  Looss 
( 1896 :192-197 )  in  attempting  to  prove  by  morphological  comparison  that 
Monostomum  verrucosum  Froel  (Notocotyle  triseriale  Diesing)  and 
Cercaria  imbricata  Looss  belong  to  the  same  species,  advances  as  his 
strongest  argument,  that  in  the  mature  cercaria  are  found  around  the 
excretory  pore  plications  arranged  as  the  rays  of  a  circle  like  those 
found  in  the  adult.  A  combination  of  adult  characters  will  often  give 
a  clue  to  the  family  or  even  in  a  few  cases  to  the  genus  to  which  the 
cercaria  belongs.  Allowance  must  be  made  however  for  the  fact  that 
adult  characters  may  be  somewhat  modified  in  the  development  of  the 
cercaria.  For  example  the  loss  of  the  tail  modifies  the  excretory  system, 
and  changes  in  shape  and  proportion  of  the  body  change  considerably 
the  relative  lengths  of  the  different  parts  of  the  digestive  system. 

Larval  characters  of  cercariae  may  be  defined  as  those  which  are 
not  carried  over  into  adult  life.  Many  structures  are  developed  to  meet 
the  exigencies  of  larval  conditions,  and  are  merely  temporary.  In  many 
cercariae  much  dependence  must  be  placed  on  such  characters  in  classi- 
fication, for  often  as  in  the  forked-tailed  and  stylet  cercariae,  adult 
characters  are  very  little  differentiated  and  the  whole  structure  is 
very  largely  dominated  by  larval  characteristics.  This  brings  up  the 
question  as  to  how  far  such  characters  can  be  considered  as  expressing 
relationship.     Like  structures  in  cercariae  either  show  relationship  or 


64  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [510 

convergence  due  to  adaptations  to  similar  environments.  Such  an  adap- 
tation as  the  development  of  a  boring  spine  which  occurs  in  cercariae 
widely  different  in  other  characters,  can  hardly  be  considered  as  show- 
ing close  relationship.  When,  however,  cercariae  are  very  similar  in  a 
number  of  larval  characters  such  likeness  can  hardly  be  ascribed  to 
convergence,  and  even  if  adult  characters  are  not  sufficiently  developed 
for  comparison,  such  forms  can  with  reasonable  certainty  be  placed 
together  in  natural  groups. 

Another  question  which  must  be  considered  in  classifying  certain 
cercariae,  is  whether  the  larvae  of  closely  related  adults  might  not  be 
different  on  account  of  modifications  in  larval  life.  From  consideration 
of  conditions  in  other  groups  this  would  seem  very  possible.  The  little 
evidence  that  we  have,  however,  seems  to  indicate  that  the  cercariae  of 
closely  related  forms  are  more  alike  than  the  adults.  Liihe  (1909:175) 
suggests  this  as  one  reason  for  the  small  numbers  of  cercariae  known 
in  comparison  with  the  adults. 

"Vielfach  sint  iibrigens  die  Cercarien  verschiedener  Trematoden- 
Arten  einander  so  ausserordentlich  ahnlich,  dass  ihre  sichere  Bestim- 
mung,  wenigstens  bei  uriseren  jetztigen  Kenntnissen,  nicht  moglich  ist, 
and  manche  alte  Art-namen  haben  dadurch  die  Bedeutung  von  Gruppen- 
statt  von  Artbezeichnungen  gewonnen." 

The  statement  of  these  problems  shows  how  merely  tentative  at  the 
present  state  of  our  knowledge  must  be  considered  any  classification  of 
cercariae. 

The  most  extensive  classification  of  the  cercariae  is  that  of  Liihe 
(1909:173-210).  His  main  subdivisions  are  for  the  most  part  based 
on  the  recognition  of  the  relationship  of  the  cercariae  to  the  larger 
recognized  adult  groups.  In  the  subdivision  of  the  distome  cercariae, 
however,  his  classification  is  to  a  considerable  extent  purely  artificial, 
being  based  on  the  character  of  the  tail.  A  summary  of  Liihe 's  classifi- 
cation follows. 

luhe's  classification  of  the  cercariae 

A.  Lophocercariae 

Cercariae  with  longitudinal  projections  along  the  sides  of  the  body. 
Ex.  Cercaria  crista  La  Valette. 

B.  Gasterostome  cercariae 

Two  long  projections  from  the  end  of  the  body.  Mouth  opening 
in  the  middle  of  the  ventral  surface.  Intestine  simple  sac-shaped. 
Ex.  Bucephalus  polymorphus  Baer. 

C.  Monostome  cercariae 

Ventral  sucker  lacking.    Ex.  Cercaria  urbanensis  Cort. 


511]  LARVAL   TREMATODES—CORT  65 

D.  Amphistome  cercariae 

Ventral  sucker  at  the  posterior  end  of  the  body.     Ex.     Cercaria 
inhabilis  Cort 

E.  Distome  cercariae 

Ventral  sucker  some  distance  in  front  of  the  posterior  end  of  the 
body. 

1.  Cystocercous  cercariae 

Base  of  the  tail  forms  a  space  into  which  the  body  can 
be  drawn.    Ex.  Cercaria  macrocerca     Filippi. 

2.  Rhopalocercous  cercariae 

Tail  having  as  great  or  greater  width  than  the  body.    Ex. 
Cercaria  isopori    Looss. 

3.  Leptocercous  cercariae 

Tail  straight,  slender,  and  narrower  than  the  body. 

a.  Gymnocephalous  cercariae 

Anterior  end  rounded,  without  stylet  or  boring  spine. 
Ex.     Cercaria  megalura.     Cort. 

b.  Echinostome  cercariae 

Anterior  end  with  a  collar  and  crown  of  spines.     Ex. 
Cercaria  trivolvis    Cort 

c.  Xiphidiocercariae 

Anterior  end  with  stylet.    Ex.  Cercaria  isocotylea   Cort. 

4.  Trichocercous  cercariae 

Tail  set  with  spines.    Ex.  Cercaria  setifera  Moulinie. 

5.  Furcocercous  cercariae 

Tail  forked  at  its  end.    Ex.  Cercaria  douthitti  Cort. 

6.  Microcercous  cercariae 

Tail  stumpy.     Ex.  Cercaria  brachyura  Lespes. 

7.  Cercariaeae 

Tail   entirely   undeveloped.     Ex.   Leucochloridium   para- 
doxum  Carus. 

8.  Rattenkonigcercariae 

Cercariae  with  tails  joined,  forming  a  sort  of  colony. 

In  the  present  state  of  our  knowledge  it  seems  to  me  that  for  the 
comparison  of  forms,  no  general  grouping  can  be  suggested  which 
will  be  of  more  help  to  workers.  It  must  be  recognized,  however,  that 
many  of  the  groups  are  purely  artificial..  The  Gymnocephalous  cercariae 
have  little  in  common  but  negative  characters,  and  the  stylet  cercariae 
form  a  very  heterogeneous  mass.  These  and  other  groups  are  merely 
temporary  arrangements  for  convenience  and  must  be  split  up  or  rear- 
ranged into  more  natural  groups  as  soon  as  our  knowledge  permits. 


66  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [512 

Marie  Lebour  (1912)  tried  to  substitute  for  Liihe's  classification  one 
based  on  other  characters.  According  to  her  classification  the  cercariae 
are  divided  into  two  large  groups  depending  on  whether  they  develop 
in  rediae  or  sporocysts,  and  within  these  groups  are  formed  small  groups 
of  apparently  related  forms.  As  an  attempt  to  form  closely  related 
groups  this  work  is  very  suggestive,  but  as  a  working  classification 
it  has  very  limited  value.  In  the  first  place  there  is  little  evidence 
for  the  belief  that  development  in  sporocysts  or  rediae  expresses  funda- 
mental relationship.  Such  widely  divergent  forms  develop  from  spor- 
ocysts as  Bucephalus,  tailless  cercariae,  stylet  cercariae,  etc.  The  most 
important  part  of  Miss  Lebour 's  work  is  her  attempt  to  build  up  natural 
groups  of  closely  related  forms  centering  around  some  well  known 
species,  as  for  instance,  her  Spelotrema  group  centering  around  Spelo- 
trema  excellens.  The  following  is  the  essential  part  of  Lebour 's  classi- 
fication. 

Lebour 's  Classification  of  the  Cercariae 

A.  Gasterostomata 

Cercariae  develop  in  sporocysts.    Mouth  at  middle  of  ventral  sur- 
face.   Ex.  Bucephalous  polymorphus  Baer. 

B.  Prostomata 

Mouth  at  anterior  end. 
1.     Distome  cercariae 

Two  suckers. 

a.  Cercariae  developed  in  sporocysts. 

(1)  Gymnophallus  group 

Cercaria  tailless.  Ex.  Cercaria  glandosa  Lebour. 

(2)  Fork-tailed  cercariae 

Tail  forked  at  its  end.  Ex.  Cercaria  douthitti 
Cort. 

(3)  Spelotrema  group 

Free  swimming  stage  with  stylet.  Encysted 
cercaria  tongue-shaped,  covered  with  spines,  with 
long  prepharynx  and  esophagus,  short  ceca  not 
reaching  to  the  end  of  the  body.  Ex.  Cercaria  of 
Spelotrema  excellens.     (?) 

(4)  Stumpy-tailed  cercariae 

Tail  broad  and  stumpy.  Ex.  Cercaria  brach- 
yura  Lespes. 

(5)  Lepodora  group 

Body  covered  with  spines.  Intestinal  ceca 
reaching  nearly  to  posterior  end  of  body.'    Tail 


513]  LARVAL   TREMATODES—CORT  67 

present  in  very  young  forms,  but  cast  off  before 
encystment,  which  takes  place  within  the  sausage- 
shaped   sporocysts.     Ex.    Cercaria   of   Lepodora 
rachiaea.  (?) 
b.  Cercariae  developed  in  rediae. 

(1)  Cercariae  neptuneae 

Tail  very  thick  and  large,  two  eye-spots  present. 
Excretory  vesicle  very  thick  walled.  Ex.  Cer- 
caria neptuneae  Lebour. 

(2)  Acanthopsolus  group 

Two  eye-spots,  intestinal  ceca  reaching  nearly 
to  the  end  of  the  body,  thin  tail  developed  in 
young  forms  but  cast  off  before  it  is  full  grown. 
Ex.  Cercaria  of  Acanthopsolus  lageniformis.  (?) 

(3)  Echinostomum  group 

Cercariae  with  anterior   collar  and   crown  of 
spines.     Ex.  Cercaria  trivolvis  Cort. 
2.    Monostome  group 

One  sucker  present.    Ex.  Cercaria  urbanensis  Cort. 

It  is  in  attempts  to  join  a  few  closely  related  cercariae  into  groups 
which  are  probably  natural,  rather  than  in  further  broad  generalizations 
which  must  be  based  on  artificial  characters,  that  hope  for  advance  in 
the  classification  of  cercariae  lies.  As  more  and  more  life-histories  are 
worked  out,  such  groups  can  be  fitted  into  their  place  in  the  adult  classi- 
fication, until  the  relationship  of  all  the  cercariae  groups  to  the  adults 
will  be  known. 


Note.    All  of  the  species  of  Cercaria  discussed  in  detail  in  this  paper  were 
originally  described  in  my  preliminary  report  (Cort,  1914). 


68  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [514 


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of  his  contributions  to  science  arranged  and  edited  by  Joseph  Leidy,  Jr. 
Smithson.  Misc.  Col.,  Washington  (1477)  46:1-281. 
Leuckart,  R. 

1886-1901.     Die   Parasiten   des   Menschen  und  die  von  ihnen   herriihrenden 
Krankheiten.     Ein  Hand-  und  Lehrvuch   fur  Naturforscher   und  Aerzte. 
1  Bd.  Abt.  2.  pp.  1-897. 
von  Linstow. 

1884.    Helminthologisches.     Archiv  f.  Naturgesch.,  50  Jahrg.,   1:124-145. 
1896.     Helminthologische  Mittheilungen.     Arch  f.  mik.  Anat.,  48:375-397. 
Looss,  A. 

1892.  Amphistomum     subclavatum     und     seine     Entwicklung.      Leuckart's 
Festschr.,  pp.  147-167. 

1896.    Recherches    sur   la    faune   parasitaire    de   l'Egypt.      Premiere   partie. 
Mem.  de  lTnst.  Egypt,  3:1-252. 
Luhe,  Max. 

1909.     Parasitische     Plattwurmer.      1,     Trematodes.     Die     Susswasserfauna 
Deutschlands,  17:1-217. 
Lutz,  A. 

1893.  Weiteres    zur   Lebensgeschichte    des    Distoma    hepaticum.      Centr.    f. 
Bakt.  u.  Parasitenk.,  13:320-328. 

Moulinie,  J.  J. 

1856.     De   la   Reproduction   chez   les   Trematodes   endoparasites.     Mem.    d. 
l'lnst.  Genevois,  3:1-279. 


70  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [516 

NlCOLL,  W. 

1906.     Some  new  and  little-known  Trematodes.    An.  Mag.  Nat.  Hist.,  17:513- 

527. 
Nitzch,  Ch.  L. 

1807.    Seltsame  Lebens-   und    Todesart    eines    bisher    unbekannt    Wasser- 

thierchens.      In    Kilian,    Georgia,    oder    Der    Mensch    im    Leben    und    in 

Staate,  pp.  257-262;  281-286.     (Cited  after  Braun.) 
1817.    Beitrag  zur  Infusorienkunde,  oder  Naturbeschriebung  der  Cercarien 

und  Bacillarien.     N.  Schrift.  d.  Naturf.  Gesellsch.,  Halle,  vol.  3.     (Cited 

after  Braun.) 
Pagenstecher,  H.  A. 

1862.    Untersuchungen  iiber  niedere  Seethiere  aus  Cette.    Ztsch.  f.  wissensch. 

Zool.,  12:265-311. 
1881.    Allgemeine   Zoologie   oder   Grundgesetze   des   thierischen    Baues   und 

Lebens.     Part  4,  pp.  959. 
Sonsino,  P. 

1892.    Studi    sui   parassiti    molluschi   di   acqua   dolce   dintorni   di   Cairo    in 

Egitto.     Leuckart's  Festschr.,  pp.  134-147. 
Ssinitzin,  D.  Th. 

1905.     Contributions  to  the  Natural  History  of  Trematodes.     The  Distomes 

of  Fish  and  Frogs  in  the  vicinity  of  Warsaw.     Separate,  pp.  210.     (Rus- 
sian.) 


517] 


LARVAL   TREMATODES—CORT 


71 


EXPLANATION  OF  PLATES 

Unless  otherwise  stated  all  figures  are  drawn  with  a  camera  lucida. 

The  lettering  on  some  toto  drawings  of  the  cercariae  was  omitted  by 
error.  The  descriptions  in  the  text  will  be  sufficient  to  make  the  struc- 
ture clear. 


ABBREVIATIONS   USED 


ab  annular  bands  of  muscle 

ac  acetabulum 

acr  anterior  collar  of  redia 

bp  birth  pore  of  redia 

eg  cystogenous  glands 

ccg  cephalic  glands 

cm  cystogenous  material 

cr  cercaria  in  redia 

dc  ducts  of  cephalic  glands 

e  eye-spot 

es  esophagus 

ex  excretory  system 

exv  excretory  vessel 

gb  germ  ball 

*  intestinal  cecum  of  cercaria 

ir  intestine  of  redia 

la  locomotor  appendages  of  redia 


Ic 

large  central  cells  of  tail 

Is 

lobes  of  snail's  liver 

»ll 

muscle  layer 

n 

nervous  tissue 

OS 

oral   sucker 

P 

pigmentation 

Pb 

pharyngeal  bulb 

PC 

parenchymatous  cells 

Pi 

pigment  line 

pr 

pharynx  of  redia 

ra 

reproductive  anlage 

s 

stylet 

sc 

cercaria  in  sporocysts 

sg 

stylet  glands 

sw 

wall  of  sporocyst 

vi 

excretory  vessels  of  tail 

72  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [518 


EXPLANATION  OF  PLATE 

Figures  i  to  4.     Free  hand  drawings  of  stages  in  the  process  of  encystment  of 

Cercaria  urbanensis.     X  about  70. 
Fig.    5.    Mature  Cercaria  urbanensis,  ventral  view.   Cystogenous  glands  not  shown. 
X  140. 

Posterior  locomotor  projection  of  Cercaria  ephemera.     From  Ssinitzin. 

Posterior  locomotor  projection  of  Cercaria  imbricota.     From  Looss. 

Posterior  locomotor  projection  of  Cercaria  urbanensis.     X  433. 

Cross  section  of  the  tail  of  Cercaria  urbanensis.    X  433. 

Immature   redia  of  Cercaria  urbanensis...  X  88. 

Mature  redia  of  Cercaria  urbanensis.    X  88. 

Mature  redia  of  Cercaria  urbanensis,  showing  constrictions.     X  44. 

Cross  section  of  immature  redia  of  Cercaria  urbanensis.     X  433. 


Fig. 

6. 

Fig. 

7- 

Fig. 

8. 

Fig. 

9 

Fig. 

10. 

Fig. 

11. 

Fig. 

12. 

Fig. 

13. 

PLATE   I 


519]  LARVAL   TREMATODES—CORT  73 


PLATE  II 


74  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [520 


EXPLANATION  OF  PLATE 

Fig.  14.    Immature  redia  of  Cercaria  urbanensis.    X  88. 

Fig.  15.     Redia  of  Cercaria  inhabilis.    X  88. 

Fig.  16.    Mature  Cercaria  inhabilis,  ventral  view.     Cystogenous  glands  not  shown. 

X  88. 
Fig.  17.    Cross  section  of  Cercaria  inhabilis  in  region  of  eye-spots.    X  195. 
Figures  18  to  21.     Anterior  body  region  of  different  stages  of  Cercaria  inhabilis, 

showing  changes  in  pigmentation.     X  140. 
Fig.  22.    Cross  section  of  the  tail  of  Cercaria  inhabilis.   X  433. 


PLATE  II 


521]  LARVAL   TREMATODES—CORT  75 


PLATE  III 


76  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [522 


EXPLANATION  OF  PLATE 

Fig.  23.  Mature  Cercaria  diastropha,  dorsal  view.  Cystogenous  glands  not  shown. 
X  88. 

Fig.  24.    Free  hand  drawing  of  Cercaria  diastropha  from  the  side.     X  75. 

Fig.  25.    Redia  of  Cercaria  diastropha.     X  88. 

Fig.  26.     Cercaria  caryi,  ventral  view.    From  Cary's  material.    X  140. 

Fig.  27.    Cercaria  megalura,  ventral  view.    From  Cary's  material.    X  140. 

Fig.  28.  Diplodiscus  temporatus,  ventral  view.  From  Cary's  experimental  tad- 
poles.   X  140. 


PLATE    III 


523]  LARVAL   TREMATODES-CORT  77 


PLATE  IV 


78  ILLINOIS  BIOLOGICAL   MONOGRAPHS  [524 


EXPLANATION  OF  PLATE 

Fig.  29.    Cercaria  megalura  before  the  extrusion  of  cystogenous  material,  ventral 

view.    X  195. 
Fig.  30.    Cercaria  megalura  after  the  extrusion  of  cystogenous  material,  ventral 

view.     X  195. 
Fig.  31.    Mature  redia  of  Cercaria  megalura.     X  88. 
Fig.  32.    Cross  section  of  tail  of  Cercaria  megalura.     X  433. 
Fig.  33.    Cross  section  of  Cercaria  megalura  before  the  extrustion  of  cystogenous 

material.    X  276. 
Fig.  34.    Cross  section  of  Cercaria  megalura  after  the  extrusion  of  cystogenous 

material.    X  276. 
Fig.  35.    Cross  section  of  immature  redia  of  Cercaria  megalura.    X  433. 
Fig.  36.    Immature  redia  of  Cercaria  megalura.     X  88. 


PLATE  IV 


525]  LARVAL   TREMATODES—CORT  79 


PLATE  V 


80  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [526 


EXPLANATION  OF  PLATE 

Fig.  37.  Mature  redia  of  Cercaria  trivolvis.  X  88. 
Fit'.  38.  Immature  redia  of  Cercaria  trivolvis.  X  88. 
Fig.  39.     Mature  Cercaria  trivolvis,  ventral  view.     Cvstogenous  glands  not  shown. 

XI95. 
Fig.  40.     Cercaria  rubra  freed  from  cyst,  ventral  view.     X  195. 
Fig.  41.     Cercaria  rubra  inside  of  cyst.     X   195. 
Fig.  42.    Anterior  end  of  Cercaria  rubra,  dorsal  view.    X  195. 
Fig.  43.    Cercaria  reflexae,  ventral  view.    Cystogenous  glands  not  shown.    X  88. 


PLATE  V 


527] 


LARVAL   TREMATODES-CORT  81 


PLATE  VI 


82  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [528 


EXPLANATION  OF  PLATE 


Fig.  44.    Immature  relia  of  Cercaria  reflexae.     X  88. 

Fig.  45.     Mature  redia  of  Cercaria  reflexae.     X  44. 

Fig.  46.     Nervous  system  of  immature  redia  of  Cercaria  reflexae.    X  276. 

Fig.  47  and  49.     Stylet  of  Cercaria  trigonura,  side  view  and  ventral  view.    X  433. 

Fig.  48  and  50.     Cercaria  trigonura,  side  view  and  ventral  view.     X  195. 

Fig.  51.     Very  immature  redia  of  Cercaria  trigonura.     X  195. 

Figs.  52  and  53.     Rediae  of  Cercaria  trigonura,  containing  germ  balls.     X  140. 

Fig.  54.     Cross  section  of  Cercaria  trigonura  in  region  of  acetabulum.     X  433. 


PLATE  VI 


529]  LARVAL  TREMATODES—CORT  83 


PLATE  VII 


84  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [530 


EXPLANATION  OF  PLATE 

Pig-  55-     Cercaria  douthitti,  ventral  view.     X  195. 

Fig.  56.    Cross  section  of  Cercaria  douthitti  thru  eye-spots.    X  433. 

Fig.  57.     Diagram  of  the  excretory  system  of  Cercaria  douthitti  in  the  region  where 

the  tail  joins  the  body.     X  about  400. 
Fig.  58.     Cross  section  of  Cercaria  douthitti  thru  the  acetabulum.    X  433. 
Figures  59  to  62.     Cross  sections  of  Cercaria  douthitti  showing  the  passage  of  the 

ducts  of  the   cephalic  glands  thru  the  oral   sucker.     X  433. 
Fig.  63.     Section  thru  a  sporocyst  of  Cercaria  douthitti  while  still  included  in  the 

snail's  liver.     X  433. 
Fig.  64.     A  portion  of  a  sporocyst  of  Cercaria  douthitti.     X.  44. 
Fig.  65.     A  portion  of  a  sporocyst  of  Cercaria  isocotylea.    X  44. 
Fig.  66.     Sporocyst  of  Cercaria  isocotylea.    X  44. 

Fig.  67.     Stylet  of  Cercaria  isocotylea  from  ventral  and  side  view.    X  433. 
Fig.  68.     Cercaria  isocotylea,  ventral  view.    X  311. 


PLATE  VII 


531]  LARVAL  TREMATODES—CORT  HS 


PLATE  VIII 


86  ILLINOIS  BIOLOGICAL  MONOGRAPHS  [532 


EXPLANATION  OF  PLATE 

Fig.  69.  Stylet  of  Cercaria  polyadena,  ventral  view.     X  433. 

Fig.  70.  Cercaria  polyadena,  ventral  view.    Cystogenous  glands  not  shown.    X311. 

Fig.  71.  Cercaria  polyadena  in  cyst.     X  140. 

Fig.  72.  Sporocyst  of  Cercaria  brevicaeca.     X  88. 

Fig.  73.  Stylet  of  Cercaria  brevicaeca,  ventral  view.     X  433. 

Fig.  74.  Free  hand  drawing  of  Cercaria  brevicaeca,  ventral  view.     Cystogenous 

glands  not  shown.    X  about  150. 

Fig.  75.  Stylet  of  Cercaria  hemilophura,  side  view.    X  433. 

Fig.  76.  Cercaria   hemilophura,   ventral    view.      Cystogenous    glands    not    shown. 

X  140. 

Fig.  77.  A  portion  of  a  sporocyst  of  Cercaria  hemilophura.     X  44. 

Fig.  78.  Cercaria  hemilophura,  side  view.     X.  140. 

Fig.  79.  Stylet  of  Cercaria  leptacantha,  ventral  view.     X  433. 

Fig.  80.  Immature  Cercaria  leptacantha,  ventral  view.    X  433. 

Fig.  81.  Sporocyst  of  Cercaria  leptacantha.    X  88. 


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